Sharing with another male character in a harem game usually doesn't go too well but good luck you're gonna need it.
To add to what he's saying, for a lot of people, sharing is pretty close to ntr or cheating for a lot of people. For most people into the harem trope, they get possessive. I recommend, that if you like those scenes, then absolutely go for it. I know for myself, personally, that's how I feel about it. Not really a fan of sharing, but hey, you did say that all 18+ scenes will be optional. (which, by the way, is a nice quality of life option, but more than likely if someone plays a "harem" game, then they're in it for the sexual scenes and the story if you manage to get them hooked.)
P.S, if you're just looking to make threesome scenes regardless of the sexes involved, then the MC and two girls or even three girls while the MC watches or something similar would be the safest route rather than involving another male.
Restating what those above said about things like sharing, since they're rather eager to encourage you to go ahead and add that type of stuff if you want to, the problem isn't when devs add in stuff they themselves are into or just whatever they decided they wanted to do in the story, it's when certain things are forced upon the player that puts some off. You can add whatever type of content you want, so long as it's 100% optional and only happens when initiated by the player, then no one should get put off. I personally don't like sharing, period. I don't like sharing a girl with another guy, or being forced to share a girl with another girl, or be forced to watch two girls have sex, or be forced into threesomes. I'd prefer that stuff be up to me, so I can just not have them happen if I don't want it. I personally prefer exclusivity between me and a love interest, though I realize when a game has harem in the title there's certain things to expect. Most harem games decide what the player is into to an extent and decide what sexual content they will automatically participate in without letting the player have a say. Most devs ignore the obvious solution. Sure, it would mean some extra work on the devs part, which is presumably why most don't always bother doing it, but if things like any kind of sharing in general with another guy or between two girls, and every fetish out there, are completely optional and only happen when initiated by the player, then people shouldn't have to feel uncomfortable, and you can continue adding whatever you want. Let the player decide what they are into, what type of content gets initiated by the player or by others, and if they do anything with any of the love interests at all. That's the obvious solution to ensuring everyone can enjoy your game and story, if you're going to be player choice oriented~