And I just finished reading a novel where that was a background plot, and as I read it, I began to feel bad - if the real young man decided he was in love with another one, it means he's either always known he was gay or bi, or loves the false young man enough to transcend that (yeah, basis of slash stories.) And so, when he discovers the truth, he's more likely to feel anger than pleasure or relief, and since the person in the boy's clothes is a different person - one who lied to him - he might need time to sort out his feelings.
I sort of covered this in the Sweetcheeks series, where Blair falls in love with the very straight-acting Jim Ellison, only to find that he's been hiding his true queenlike self. I wrote it for laughs at first, but by the end, I knew it was a real problem that would have to be addressed.
As it happens in the novel, the real young man figured out that it was a young woman *first*, and then fell in love, which was perfect. He also understood that the reason she lied was to get the sort of job she loved but couldn't get in that medieval fantasy setting as a woman. In fact, when they married, it was to form a partnership using her considerable skills in that area. All good, you see.
I also watched Victor/Victoria yesterday. And - aside from the fact that no one could look a Julie Andrews and not see and hear "woman", even with short hair, men's clothes and a deepened voice - it was the same plot. And James Garner, who had led himself to accept the fact that he was attracted to a (extremely feminine) man, to the point that he was spying on 'Victor" in his bath, was entirely relieved, despite the fact that he'd possibly been assuming fantasizing about an very different set of parts.
Of course, "Yay! I'm not gay!" is probably a reasonable reaction for that particular character in that place and time. Or even later, thinking about Chris in the Morning on Northern Exposure and his crush on the monk who turned out to be female.