Ahhhhhh this is SUCH a big question!
What you’re describing is a very common and very loved trope in M/M romance: “Gay For You”. People eat it up, but there’s a lot of criticism of it, too. Here are some queer men and womens’s responses:
Damon Suede: http://www.reviewsbyjessewave.com/2011/07/22/gay-for-you-or-out-for-you/
Josephine Myles: http://josephinemyles.com/2011/02/22/trope-tuesday-gay-for-you/
Alexis Hall: http://www.quicunquevult.com/blank-for-you
Here are my individual issues with the trope, as a bisexual woman:
1. When it gets entangled in misogyny—ie, when the new gay relationship shows the GFY character that women are whores and sex with them is meaningless and loving them is quaint and silly compared to the Powerful and Deep Connection Between Manly Men. Sometimes non-monosexual people people experience attraction differently depending on the gender of the other person. Sometimes strictly homosexual people try at heterosexual relationships for myriad reasons but it’s not “right” and those relationships, while still valuable, aren’t fulfilling for them in the same way since they don’t truly align with their orientation. This is all fine and normal. However, when we try to make a universal statement: “relationships between men are always more pure and special and valid and equal than relationships between a man and a woman”, that sends a really terrible message.
2. When the GFY character has expressed attraction to women but suddenly upon meeting his love interest flips like a light switch to full on gay, aka, the character he’s attracted to “turns” him. So here we have a guy who has dated girls, slept with girls, been in love with them, etc. but then he meets this one man and suddenly girls hold no interest whatsoever. He’s Gay, fully homosexual. Like his sexuality is a light switch that flicks from Straight to Gay all because of one guy. People can be deeply in denial and deeply in the closet, and maybe they were Gay All Along, but having a character actually genuinely change from straight to gay … not so great for the most part, especially since this aspect of the trope not only erases bisexuality, but also often ties in with point number 1 above.
3. Using the GFY trope as a way to write gay sex about “straight-acting” men. There’s a THING, both in gay culture AND in female-dominated M/M spaces, that basically denigrates queer men who are in any way effeminate, or campy, or visibly gay in any way. You see this on sites like Grindr where men proudly announce they want “straight-acting” sex partners, but also in M/M where characters who don’t meet some criteria of masculinity/outward heterosexuality are called out as “chicks with dicks”, thus implying that their stories aren’t worth telling and that they aren’t deserving of love or sex and are basically not the Good Kind of Gay. GFY as a trope, then, fulfills this worship of straight masculinity by transferring “straight acting” to actual “straight”. So the reader/author gets their jollies writing/reading gay sex and gay romance, all while still getting to uphold the dominance and idealization of heterosexual masculinity, which is incredibly toxic and harmful for any man (queer or no) who doesn’t fulfill that ideal.
4. Completely disavowing the possibility of bisexuality. Not to say that there aren’t real people in the world who don’t want to label themselves, or who really do consider themselves straight or gay but with an exception, but in fiction this scenario is way, way, way more common than I think it is in real life. Plenty of non-monosexual people are more than happy to say “I’m bisexual” or “I’m pansexual”, but in fiction these people don’t seem to exist. Which is not to say that your character needs to go from 1. initial attraction directly within two paragraphs to 2. confidently announcing his bi/pansexuality, because that journey from one to the other is genuine and valid and also a fantastic source of tension and conflict within the narrative and can be an important part of the character’s arc. But don’t be afraid to use “the B word” at /some/ point. Ultimately, you have to do what’s true for your character, but do consider as the author that one narrative is both harmful and constantly reinforced, and the other is not seen nearly enough.
Hope this helps bb! And good luck with your book!
All of these. ALL of these. But especially point #3.