For the vast majority of it, MC is the only man having sex with the women in the harem, occasionally the women in the harem get frisky with each other but this usually leads into them inviting the MC for a threesome/orgy.
The only exceptions I can remember involve succubus characters, they are not afraid to use sex to further MC's objectives so they will sometimes sleep with people outside the harem, men and women, without the MC being present or involved with the scene. The MC is not a jealous or overbearing person and he understands succubi as a species treat sex in a different way than humans do, so this is not an issue.
The only scene I can remember where the MC is present and "shares" a woman, is a threesome where MC and a temporary futanari double-team a succubus that recently joined his harem.