... the manga spells it out a bit more. Of the species introduced so far, from what I recall the only one that's sticking around that
didn't kidnap or otherwise acquire human males for part of their breeding cycle is the dullahan (and I guess the slime). Even the centaurs did, and they're
not a monogendered species.
The lamia was actually sent out into the world to bring back a human to be
shared among her entire colony, as was culturally normal for their people until the human world in general found out about monster races and started putting feet down. Harpies would regularly just flap around and pick up whatever caught their fancy and fly off with them (and then later forget who they were and go screw someone else -- papi and her mother are apparently unusually intelligent/possessing of good memory for a harpy). The centaurs... well, they didn't start
breeding much with humans until recently, but they've been using humans as part of their happy fun times for longer than that. Arachnae did what you'd expect from a spider species. It's implied pretty heavily (where it's not outright stated) that consent isn't particularly involved in any of it, heh.
It's... the thing about the species backgrounds in monster musume is they're pretty much all porn plots. Lamia's a harem/gang-rape, harpy's a one-night stand, centaur's NTR, arachnae's black widow, etc., etc., etc. It does actually flesh them out somewhat, but... yeah. Is one of the parts about it that is fairly amusing, to me
E: Also, you'll find as you go along that centoria's spiel is not exactly normal for a centaur. Not sure if the anime has hit it yet but the manga has. There's some stuff going on there, and what she's doing is unusual for the species.