I think what he's getting at is one of them is destined to rule her people, if he gets with her he will be expected to be there with her at all times.
It won;t go down well with her people if he sleeping with women the Amazonians deem as inferior and that's assuming they accept him sleeping with anyone else at all.
It won't look good on her and she'd likely be ousted and lose the trust and love of her people. In that case would be better off leaving him when she goes back.
It's one thing wanting a harem ending it's another having make any sense at all. Some women may have to be kept out of it.
Also remember the dev has no say on Alvara, that's ALL done by his girlfriend and if she says Alvara gets a solo ending then no amount of complaints on the forum will change it, her word is law.
I...I was mostly joking at that, but yeah you have good points, my only counter point would be, there is no logical reason for them to expect that of him, even less so for Alvara to expect that, hell the MC probably will have little say on what is gonna be his responsibilities from now on from the look of things and if he gets to have a say he will be literally god-level so there is that, also I doubt he would just up and leave his city not matter the reason, after how much he fought for that place it would make no sense to leave, as much as the Amazonians would expect whatever they expect the MC has his own duties(mostly self-imposed) and aspirations and any partner worth of having wouldn't try and make them give up on that, but them again something might happen before the end that changes all that the MC is and wants so yeah that could be an ending, and again I wasn't saying that he would have a harem, I was talking about polyamory a type of relationship where Alvara would also be able to have other people in her life romantically, so it would be hard to argue that he was "lessening" her in any way, even more so if he chooses her as a primary and has that type of polyamorous relationship, but again I see your point and I was mostly joking in that quote.
Oh but that gave me another idea, clones, can he make clones? that would solve the issue, it would take a lot of power but it could work right?