How can I justify a choice that seems too binary?

Question

My universe is a fantastical universe, and the area is a Viking area. My three characters are :

• S, a character who was born female but raised to think they are a man to protect them from their father who is obsessed with sons. S is torn apart by their secret, has to live with their awful father and twin brother, and try to protect her younger brothers who are shy and submissive to their father and her twin.
• A, S' first love who is married to their awful brother, mother to his children. They wanted to be married to S but had to marry the first-born but still doesn't know that S was born female and when she learned it, she reacted pretty badly. She can absorb and release strength, pain and injuries to cure or kill someone.
• M, a mermaid who had a crush on S when she saved them from drowning, she is born from a magical spell and can't have biological children in order to avoid creating humans born from magic.

My idea was to separate S and A because A can't understand S' identity, while M is also born with a different identity.

Then A's kids become sick because of a mistake S made, and A absorbs all the energy of M to cure her kids through her magic. It kills M, and S is destroyed by M's death and can't stand seeing her former love and her nephew and niece who are alive because of M's death.

My question is : How can I justify this choice without going for the full "A is an old fashioned lady and M is the modern girl" ? Outside of these events, I want A and M to be close (because yeah M will come back from the dead), M to help A when she's doubting, helping with her kids, A to care for M when S is away. But S still has a choice to make, and I want it to be felt as the logical end of their love story with respect and caring rather than a hateful moment, that M will understand why A made this choice and A to feel that M is the best choice for S.

(Hope it doesn't seems like I am asking for creative ideas like it's forbidden, I really want to improve my style and how I write.)

Answer 1

It doesn't have to be the case that A is "old fashioned". A's motivations can be related to love/caring for her family, wanting to protect her family, being worried about how a relationship with S would impact her family (and the associated reaction of her husband, S' brother). These are not cases where A made the choice out of some bias or discrimination towards a "different" or "non-traditional" relationship but in order to protect her family and/or herself from dangerous influences.

This naturally extends equally to the decision to kill M in order to save A's children. It is a decision made of love and not bias or discrimination. It is a case of priority for the character that results in pain for S.

Answer 2

A isn't Old-fashioned, she's just heterosexual. S and M are not "modern", they are just homosexuals. Both have existed for thousands of years. A "reacted badly" because she thought S was a man, and discovered differently, and that she'd been deceived by S all along. Lots of homosexuals fall in love with people that are not sexually interested in them, just like lots of heterosexuals fall in love with people of the opposite sex that they don't have a chance of ever realizing.

S moved on the M, the magical mermaid. When A's kids get sick, she makes the only choice she can to cure them: sacrificing M.

You say M comes back to life. Perhaps the scene you need is for A to not know that M and S are a couple, S keeps that a secret. But then A finds this out, and instead of reacting with disgust for S, she reacts with compassion: She absorbs energy from S, and herself, and it is A that brings M back to life, for S.

So we the audience know that A's reaction to S coming out to her was never about homophobia, it was about being deceived and lied to. But she forgives S, and tries to make right what she took from S to save her children.