Why do people always do this...? Why do they always have to force a lesbian character into a straight scenario? Isn't the whole point of them being a lesbian is that they don't like men?

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.