The "harem demographic" as you put it, likes a lot of women for one guy (or the reverse). At no point in time does that exclude them playing with each other - they are not mutually exclusive.
If your "group" (?) as you put it doesn't like it, that's fine. The amount of girl-on-girl play in the game really isn't that much from what I remember, at least not compared to other games.
Finally, your harem in this game consists of girls you've tamed. You can do that different ways: girlfriends (who still exhibit lots of free will, but you are their main), slaves who do your every bidding at every command, and drones who you mindwiped and who don't even respond, but still let you do whatever you want to them.
From that perspective, it's VERY easy to avoid any girl-on-girl. Make your entire harem either slaves or drones, and you never have to worry about them again.
Threesomes (as a side point) aren't a lesbian activity if the girls don't engage with each other. But the extent of that engagement is up to the creator. I would think you'd be able to avoid the threesome scenes if you want, with the exception of your mom/sister since that's required to bring them into the harem fully. Again though, I don't recall if they engage each other.
All in all, it sounds like you can avoid whatever you want to. I'd suggest instead of going to war with the forum members, you yourself actually play the game start-to-finish and see for yourself.