Dave seems to be saying that there's too much individual variation to generalize.

I don't quite think I'm saying this.

I am saying that there are people who, for the most part and all other things being equal, are better off (which is similar to happier, I guess) having multiple romantic partners, and there are other people who (ftmpaaotbe) are better off having single romantic partners. (I also think, though I haven't previously said, that there are people who ftmpaaotbe are better off having no romantic partners.)

But if you insist on asking whether people are ftmpaaotbe better off with single or multiple partners, without reference to which type of person, I do think the question is answerable. I'm not sure what the answer is. I just think it's the wrong question to ask, and I don't care very much about the answer.

This is in a similar sense that I can tell you what a person's chance of getting pregnant after unprotected sex is, independent of their age or gender, but it's really a far more useful question to ask if I break the results out by age and gender.

And, yes, I agree that ftmpaaotbe conceals a wealth of trickiness. That said, "this is a bad choice because it's socially unacceptable" is also a very tricky line of argument.