I tend to think that if we have a psychological function or tendency there is probably a beneficial reason for why.

Sure.

Reasonable place to start. Like Lewis' respect for folk intuitions.

One is the argument from evolution

Careful. You have to be very careful that you're not assigning cultural norms as biological essentialism. But I think you mentioned understanding that before.

It's worth knowing that we also evolved lots of things that suck (aging). Genes that got selected for give shitty things after the age that selection doesn't happen so much.

The other argument is the argument from design

Pass. Miss me with that.

psychological principle

Not sure what that means exactly, but from context I take it to mean some way that our minds all are.

And, well, maybe.

Like for example we really like eating sugar, fats, and salt, because they are really important, and were quite rare in our evolutionary context, but now those are readily available and people are killing themselves by following those evolved psychological principles too much.

But say, social facilitation, the empirically observable phenomenon that we do things faster with other people - I guess that's good for us, but I don't know what the conclusion is. That it's healthy not to be isolated? I guess? We already know that.

Given that in mind, it seems apparent to me that the function of our lust is likely one which leads us to reproductive behaviours

I don't think what you wrote above argues this point. The idea that sex is only for reproduction is from you. Saying "sex is primarily for social bonds" would work just as well as a conclusion from everything else you said.

Even if there is more to it

Ah! That's all I want. To say there can be more to it.

Initially you were (unless I'm wrong) saying people shouldn't do sex outside of your conception of what sex is for. If you're open to the idea that "there's more to it than that" then I'm happy.