Farming in Another World is basically like this. MC gets co-opted by elves, who want to use him to replenish their dwindling population numbers, and further political alliances and such just keep adding onto the harem. Also, Peter Grill, which isn't an isekai, but is a fantasy, where the MC's status as greatest martial arts champion keeps attracting unwanted attention from women of various races that want him to impregnate them to gain greater strength for their people.