Ha, well my opinion isn’t based on gaydar or anything, just close observation of the boys. But:
Liam: based on the high number of homoerotic things he’s said and done, almost certainly not straight. Anywhere from bi to gay.
Zayn: probably bi, based mostly on how physically affectionate he is with the boys, but especially with Liam whom he acts like he has a crush on.
Niall: has said and done a couple of homoerotic things, but probably almost completely straight.