"Ugh. Nude beaches are full of overweight, out-of-shape old bodies and I don't want to go there and realize I look exactly like that."

Bobs Burgers - s03e11

spot on. And that goes double for swingers clubs.

The primary purpose of a nude beach is not to look at other people naked, but to enjoy being naked yourself.

the primary purpose of sex is procreation. Doesn't mean it isn't more appealing to do it with some people than others.

Pleasure is the primary purpose of sex for the vast majority of sexual encounters, including the majority of living people who aren't able to create biological children (due to age, sexuality, medication, or medical issue).

Arguably the procreation part is WHY it's so much more appealing to do it with some people rather than others.

If you (especially as a woman) are going to get pregnant, better make sure the father either is likely to have good genes, social leverage or will help you raise the kid, or ideally all 3.

Similarly (but less strong) for men, better make sure the woman is likely to get pregnant (she's not too old), is healthy enough to take care of any baby born (she doesn't have obvious physical defects).