Nobody is forcing nothing. It's just fanart, something that has nothing to do with canon or something official. If somebody could be threatening Tachi to make Sakura Trick a hetero work, then you could say that people is forcing that straight scenario you're saying.
Why do people always do this...? Why do they always have to force a straight character into a gay/lesbian scenario? Isn't the whole point of them being straight is that they don't like members of the same-sex?
It works both ways. That's just fanart, man. Artists will do whatever they please.