Would the route end if you chose not to do it?
Wait, are you just locked out of a threesome if you don't want lesbian content or what's the deal there? EDIT: That does seem to be the case. Personally think that's dumb, they can just become close enough to be comfortable enough for group sex without that. Frankly if one is staunchly against lesbian content too much happens implicitly before you even have the choice to say "no" to Oni and the dialogue around declining her offer is patronizing. "What if they become lesbians and leave me out?", you know some people just aren't into it and it's not about competition, right?
There can be group sex without turning the girls lesbian, and you can still include girl-on-girl moments within the threesome. The moment the girls start liking each other and start dating, it ceases to be a harem but becomes a poly. It's honestly annoying how often the "harem" games I've played turn into poly, and the lesbian relationships end up becoming the focus of those games and the MC is fallen by the wayside.