The gay romance genre comprises a number of sub-genres. The usual sorts, such as fantasy, science fiction, historical, and so forth. But one of the sub-genres unique to m/m romance is what's sometimes abbreviated as GFY - Gay For You.
In a GFY story, the characters were not previously gay, or at least didn't know they were, until they met that one special someone and embarked on a gay relationship with them. The implication is usually that one or both protagonists wouldn't be in a same-sex relationship except with this one other person. Because they're Gay For Them.
Personally, I'm fine with GFY stories, but there are lots of readers who really don't like this particular story trope.
I've seen "Zero at the Bone" referred to as a GFY story a number of times. I never thought of it that way myself. I can see how it could be considered as such. Neither Jack nor D has really ever had a long-term or significant relationship with another man before. Both were at one time married to women, although both relationships were over long before they met each other. Jack is described as having had at least physical relationships with men. D hasn't, but he's had romantic and sexual feelings for men before.
So in my mind, it was clear that they were both gay before they met each other. They just hadn't had a gay relationship before. That could be considered GFY. But they were both gay for more than just each other, so using that definition, it wouldn't be.
What do you think?