Because BioWare has put same sex romances in every game they've released with a romance option since Dragon Age and, this may come as a shock to some people, but sometimes gay people like to role play as gay characters in role playing games. And, really, BioWare has no one to blame but themselves. Before Dragon Age I was totally fine with hetero romances but once they started putting in gay ones I was hooked. I can never go back. So, yeah, that's pretty much why.