I mean, incest wouldn't be sustainable, so I really hope MC isn't actually the last man... Otherwise the whole plot only slowed the extinction of humankind.
Indeed, MC's harem is about the limit for genetic diversity using only one dude, I believe. Diminishing returns since everyone's half-siblings means you can't really get beyond four... I forget what the actual figure is called. Genetic Diversity Points, fuck it. Point is, since everyone shares a daddy, no amount of arranged marraiges and genetic lottery will preserve the species, you need a good 200 guys to do it even with infinite bitches.