The child is better off than he would have been had he been left in the subway. Kids need most of all someone who will go out of their way to keep them safe and healthy.
He had parents who were presumably a man and a woman. They left him in a bundle in a subway.
He had parents who were a man and a man. They dropped everything in their life and rearranged it so they could save a stranger's baby who only needed to be taken care of.
What's the point of even questioning his sexuality? He's prepubescent, he's not got a problem with his sexuality because he doesn't even have a sexuality.
I do know he will have grown up with parents who were not 'normal', but who were tolerant enough to sacrifice their whole quality of life for the baby of a stranger, of another race, of whom they knew absolutely no history. He could have had fetal alcohol syndrome, he could have been a crack baby, he could have been broken in many imaginable ways. Instead, they took this baby, they stood by it even when the 911 operator didn't believe their story, they made sure it was looked after, and when asked they agreed to raise it without hesitation.
These are upstanding people. Nothing indicates that homosexuality is contagious, but even if the kid does end up gay due to his upbringing, then what of it? It didn't stop his parent's from being upstanding people. Being straight didn't make his birth parents paragons of virtue.