I know that lots of people are angry about women writing those types of characters. I'm not trying to appropriate or fetishize anyone
There is no clear line here, but you can always "test" your characters and scenes in the same way male authors are encouraged to "test" their female characters – ie: do they have agency, is their sexuality in service of plot/character or just titillation, are they stereotypes/monotypes, is one "good" (rewarded for conforming to society's standards) and the other "bad" (punished for non-conformity), do they talk about anything outside of who they are dating or in love with (Bechdel Test), etc.
I could change one of the character's to female and make it work though I would have to change the plot quite a bit, and both characters are based on 4 guys I know, but maybe I should just to avoid pissing people off.
I understand these are not the main character, but supporting characters, so...
- How much would it change the plot to keep them bisexual and change both to women?
- What if they are bisexual male and female, but their "normal" is
homosexual (a sub-culture or neighborhood which existed in the late
1960s in many cities)?
- What if one is homosexual, the other bisexual, and this causes
ideological problems within their relationship?
- What if they both start as bisexual, then drift towards homosexual
and heterosexual respectively?
- What if one is radical, the other conservative? What if one is out and the other doesn't care to be?
- What if they are different races, religions, or economic class?
I think you get the idea. There are endless ways you could alter them. Each would require a little re-tooling, but does it make any difference to their actual character – what they think and how they approach problems?
The truth is, we don't have any information about these 2 other than they fall in love, and eventually have a coming out scene to their mothers. Needless to say you are going to have to give them more substance than just those clichés, which are modern interpretations of "normal" for homosexuality-only. Why would bisexual men tell their families in the 1960s, when homosexuality could get you arrested, fired, and even force-medicated? Would they even identify with "out" homosexuals, or would they consider themselves a kind of "free love" that doesn't need to be defined?
On a side note, plenty of witch cults and pagan temples were de facto bisexual, at least ceremoniously. Sex magic was just as big in the '60s as any other decade.
I think your heart is in the right place, but based on the brief description I can see where some gay and especially bisexual men would feel you are not representing them as real people, certainly not behaving realistically for the time period.
Create the characters first. Understand who they are and what motivates them as people – especially what makes them different from one other – and then build the relationship between them.
If your characters' arcs can survive gender/race/sexuality/class changes (with minor adjustments) and their motives/actions still make sense, you have nothing to worry about because they are real people first, whatever the gender or sexuality.
But if the character arcs fall apart by changing their gender/race/sexuality/class then you might have shallow stereotypes who are dependent on racist/sexist tropes.