Proper long romance or only in threesome scenes where mc will be presented, and player can either encourage girls to play with each other or "forbid" It.
If you're thinking of some girl suddenly going lesbian for w/e reason, then yeah, that can be made optional. Altho I'm not really planing any content like that. At least not with deep emotional romance. Two girls experimenting and having casual fun with hero in a threesome is a different story.
There is two lesbians that the hero will ultimately get to sleep with, but he won't be taking much interest in their love interests. Also, they are optional as all other side characters. He'll probably convert and set them straight too.