Well, there's
Lust and Power. You will see a bit of ero-content regardless (generally not with the MC), but the vast majority can be done at your own pace.
I tried to think of games that have some kind of gameplay + a literal harem-type thing, with some detachment from the rest of the game. But most do have unavoidable sex, at least when inducting a new harem member. There is
Ravager, that one lets you take your time with some of the girls (and avoid/eat others). Also
maybe Lord King, but I haven't played it long enough to know for sure.
With a caveat, I can also recommend
News Desk, because the sex is separate from the core gameplay. The caveat is, the two are connected in that sex progress with the two main characters will affect the game, making it easier as you progress.
But a lot of the early interactions have more to do with humiliation/domination rather than sex, so you may choose those and still benefit.