I have two female characters, one who is lesbian, and the other bisexual. However, I am male. So not only am I to write the opposite gender, which I actually feel more comfortable doing, for some odd reason, but also a homosexual relationship, which I attempted once. So, how do I write it to prevent flak?
You might be overthinking it.
What I mean by that is not to avoid or whitewash the differences implied by gender or sexual orientation. What I mean is that you're writing a romance.
Anything particular to a lesbian couple is more going to be defined by their environment. In a society that has functionally abolished heteropatriarchy, they won't be a 'lesbian couple'...they'll be a couple.
So maybe start with your character's background. Where they grew up, their levels of affluence, education, etc. Then add gender. How does this clay-sketch-of-person change under whatever gender role pressures exist in your setting? Now you have a male or female or nonbinary character, that is specific to a setting and background.
When that's done, consider sexual orientation. How does being gay change this? Is it accepted? Taboo? What is risked by expressing this? What has your character been taught about sexual orientation and how do they feel about it?
Do this for both characters and then put them together. How do they react to each other? Do they even like each other? Your characters aren't just going to automatically fall in love because they're both gay. You might find, when you flesh them out, that they're actually incompatible and you need to go back and rework who they are and how they came to be that way.
This is all to say that there aren't special rules for LGBTQ characters or women. The same considerations you have when you develop them you should really be applying to all your characters.
If you're writing a standard cishet romance, you're not just writing "a romance", you are writing a romance with two cisgendered heterosexual people. That's not some baseline, or a neutral place, it's a specific kind of romantic pairing with it's own pressures, rules and privileges.
Write strong characters grounded in setting and upbringing. Everything else will flow from that.
Personally, I think the emotions of love and romance are one thing and not gendered, that if we could discard the cultural baggage of what other people and institutions expect of us and think about us, we would find a commonality in what it feels like to be sexually attracted, infatuated, fall in love.
Here are 11 scientific signs of a person falling in love that are not gender specific. That is not a comprehensive list but it is what I could find quickly; I have seen longer ones backed by science.
This list is missing contact craving, the desire to be physically close and touching the person you love, which is why we see people in love sitting beside each other when across from each other would be much more convenient; and holding hands and walking with arms around each other despite this being less efficient than walking beside each other without being physically connected.
It is also missing a distorted sense of symbolism in worrying about or obsessing on the cosmic meaning of small gifts, or comments by your loved one.
Here are some good notes on The differences between lust and love, which will help to inform your descriptions of love. The main difference? Lust is centered on physical aspects and imagining the sex itself. Love can contain lust, but is centered on connection, the desire to meet their friends and family, to share secrets, hopes and dreams, your own and theirs, to share a future life, to join in and share an interest of your partner's and adapt yourself to fit in their world. If they love you, this will be reciprocal. If it is lust, they are not truly interested in what you think, feel and dream, except to the extent it is necessary to make use of your body.
So feel free to google your own symptoms or signs of love, and pick three or four you think can show in actions to illustrate this lesbian romance in terms that do not depend on gender, or on orientation.
The public activities of homosexuals (if not prohibited by culture) are not different from heterosexuals. Holding hands, hugs, kissing (contact cravings), loving eye contact, etc.
Actual sexual conduct doesn't have to be much different either; homosexuals seldom do anything to their partner that could not have been done by a person of opposite gender. The exception might be same-gender genital contact; but I don't see that as a necessity, even if you write explicit sex scenes.
I believe infatuation, sexual attraction, falling in love and being in love can be written without dealing with gender specifically; other than any cultural difficulties the lovers encounter. And whether they do have cultural barriers to overcome is up to you as the author.
Honestly, it should be easy for a straight guy. First, pick one of the couple who is the more main character... from which side do we experience the relationship (if the two are both secondary characters, which character is introduce first? Use her.) Now, write her experience in the relationship as if you were courting the other member of the relationship. What's your play? How do you ask her out for a first date? Where do you go, what do you do? Is it a good date? Does it go south but there's enough that a second date is agreed too?
Why do you like this girl? What interests do you share? What is she into that you aren't? What about the reverse?
Walk yourself through an imaginary date and relationship. And once that's all done and you reach the story's finish, go back and turn your half back into a woman.
There's a lot of these "How do I write an intersting member of group x when I am not a member of group x?!" on this SE. It's annoying to me, because you're approaching it from the wrong angle. Focusing on a character's sex or gender or sexuality or skin color, or religion, or cultural background is like trying to buy a birthday gift for someone and your biggest concern is will they like the wrapping paper... yeah... it should be appropriate. But you should be more focused on making sure the person will like the gift!
One thing concerning me is that you haven't told me anything about the characters beyond they're in love or what specific issues you're concerned about in depicting their relationship. All I know is they're lesbians, you're writing to bring attention to generic LGBT issues, help me not be controversial. What's controversial about a Lesbian and a Bisexual woman falling in love? That's Willow and Tara from Buffy the Vampire Slayer, a show from the late 90s-early 00s. The biggest controversy of the whole thing was that Joss Whedon dared to kill off Tara because he knew the fans almost universally adored.
Flash forward 20 years where the biggest controversy regarding fictional gay characters was J.K. Rowling not having the courage to show Dumbledore at least acknowledging that he and Grindlewald were in love on screen but insisting that the relationship was steamy hot (yeah, yeah, you better make Dumbledoor the Top. I don't care if Grindlewald is a power bottom, Dumbledore is the Top). Nickelodeon had the guts to show Korra and Asami holding hands like lovers in the final scene of megapopular kids franchise Avatar. And the people who are complaining (and I haven't heard heard many people complaining about Avatar, and certainly the only complaints about the Harry Potter stuff is Rowling won't... hehe... go balls deep in her scripts). The point is you're gonna get people who hate your story because "ew teh gays" and because "ew, not gay enough", and because "ew, het male lesbian porn fantasy", and "ew, it's been done", and "ew, it's not the greatest literary work of all time, but the lesbians were cute".
Screw your courage to the sticking place, write your story your own way and do not compromise on your vision. Writing by focus group is no way to go through life. And sorry if I come off as a litter ornery in my response, but I'm a Bi man, but who I choose to find attractive doesn't define me, nor do I let it.
I understand, because I've tried it too, but I am gay (I'm a man). I tried writing a story about lesbains once, but I felt a bit out-of-my-zone during that time and couldn't finish the first chapter. (I'm better at it now, so I'll try again some time)
One, read lesbian stories. Also, research them.
Two, try to understand women more as well. Both male and female authors often have trouble writing the opposite gender. I don't know how many times I've felt something off about a male character that a woman wrote (with only 1 story being "authentic" and 2 coming close).
Three, what do you mean by "a homosexual relationship, which I attempted once." Do you mean that you've actually tried physically being in one, or tried writing one? If you mean tried being in one, then I have to tell you that gay male relationships and lesbian relationships are very different. Gay men have extremely worse body issues (lesbians are the least likely demographic to have body issues, while gay men are the most likely), there's more sex (usually) in a gay male relationship, and there's more domestic abuse in a lesbian relationship (lesbians are the most likely demographic to domestically abuse an intimate partner in some way, shape, or form, while gay men are the least likely demographic).
Just do some research and practice! Once you've got it down, you should do just fine.