Your question is absolutely valid, because people who are not familiar with that particular idol subculture may have an impression that most of the boy band members are gay.
2PM most probably are straight (and if not, that is not my business), however, all these couplings you see are part of fan service and fan culture. I’d recommend googling such terms as “yaoi” or “slash fiction”.
The entire phenomenon might as well be used as a base for a PhD thesis. Actually, it’s peculiar how for the most part boy band homosexual fan fictions and couplings are created by women for women. I don’t really know its appeal, because I only play with ships and don’t go down with them 100%.
However, do fans realize that the couplings are not real? Of course they do. Some K-fans even get annoyed if the boys are told of couplings face to face (even though the “homo coupling” as they call it subculture is probably the fiercest among Korean fandom). They know it’s just for fun. If anyone thinks it’s real, then I dunno, someone pop his/her bubble.