I'm with the bunny on this one. It'd be kind of a lame hook if he ends up being straight. If the premise isn't attached to the story, it's kind of disappointing. Based on the cover, I'm thinking either complicated love polygon or polyamorous relationship between the two guys and the girl.
As for the absence of the yaoi tag, that kind of misses the point of the tag, I think. The story seems oriented around the girl(rather than having multiple protagonists). It's her story, so it wouldn't be tagged yaoi just because she encounters a gay guy or a gay relationship. I would expect a yaoi tag for mmf polyamory, though, so that implies it isn't that idea.