I think that the insofar is probably not, in fact, very far. The psychological mechanisms built up around sex predate the human species, they're not going to change so easily.

Well, other humans societies are known to be more relaxed and permissive about sex than the modern Western world. And that's without effective contraception. So we clearly can improve somewhat.

Is making people more adapted to promiscuity the most helpful psychological alteration we could be making?

No. But down that road lies the argument of "invest all your effort in the single most efficient charity to the exclusion of everything else". Most people don't actually do this, so it makes sense to talk about other things too.

The suggestion here is that making people more adapted to promiscuity the most helpful alteration we could be making with regard to promiscuity. Maybe one of the most helpful alterations with regard to sex in general.