One of the immediate reasons for not having sex with everyone is because there is a cost associated with doing so!
If you accept that the idea of "life" is to propagate your genes, and thus, you want your genes to be the most fit they can be, then making a pairing with a "low-quality" partner will hamper your genes in the long run!
While the cost of sex is comparatively low for males physically, since sperm is a relatively "cheap" gamete, it is quite expensive for females! Once impregnated, a female, as everyone knows, has to gestate an infant for nine months! That’s nine months of moving slower, needing to be more careful, and then however many years to raisea baby into adulthood!
If you mated with everyone, you would need to hope that random chance slotted you with a great partner! For those on the extremely genetically "unfit" spectrum, random mating would be great, because usually, they would mate with higher quality partners!
Those on the other side of the bell curve would, by the same rationale, do worse, as they would typically end up mating with lower-quality partners.
If we’re just talking about sex for pleasure, that’s a slightly different story. But similarly, every mating with a lower-quality partner would mean you weren’t having sex with a higher quality partner. This could lead to reduced quality elsewhere besides children. Perhaps "quality" in a mate is governed by resource giving, or attention, or help with social problems or tasks.
In reality, we don’t operate like bonobos simply because we’ve evolved under different circumstances, so it’s difficult to simply rewire our brains to behave as such! Even if we don’t like to think so, there is a nearly constant assessment of "quality" going on in our social interactions.