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.

Does it make them happier? How do we know this actually constitutes an improvement?

ETA: We would have evolved different psychological mechanisms around sex if the biological and ecological conditions around it had been different millions of years ago, but those psychological mechanisms are adaptations for our genetic continuation, not our happiness. Just because we've got safer, lower consequence access to sex than in our ancestral environment, does not necessarily mean we'd be happier if we adapted to use that access to a fuller extent.

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.

We don't want to go down the road of "invest your money in the Society for Prevention of Rare Diseases in Cute Puppies" either. Lots of people do that, but that doesn't make it sensible.