This is where I have to remain idealistic. I'd much rather have no relationship at all than one in which the woman's constantly pining for some other guy.
In any case, I'd dump a girl on the spot if I found out she'd had 20 partners, so that particular example's not very relevant to me.
You're conflating concepts that don't relate. I dated a girl once who was pining for some other guy and who wasn't promiscuous. I dumped her. Likewise I dated a girl who had had better sexual partners than me who did not pine for any of them. The relationship lasted a while until another unrelated event caused it to split.
I've dated girls and had sex with them and dated girls and didn't have sex. I've seen all of these combinations, and it really looks like you're taking shortcuts that don't always hold up.
You've got your cause and effect mixed up -- in my opinion only, of course.
The most risky relationship? One with someone who had never had sex or made any "moral" mistakes at all. Because everybody has to screw up sometime. I'd much rather have somebody who has sewn their oats and learned from it. To mangle an old phrase, if you're a kid and not promiscuous, you have a problem with your heart. If you're an old person and are promiscuous, you have a problem with your head.
Sex isn't the Olympics. You're not always going to hook up with the best person and if you find a new person move on to them. Looks aren't everything. As Ben Franklin observed about having sex with older women "All cats are gray in the dark"
I've been in relationships with girls who had multiple partners who thought this way. They were sluts. (same goes for guys on the other side of the deal). I've also been with girls who had multiple partners who were extremely loyal.
Yes -- people who are uninhibited, risk-takers, and have poor judgment skills are not the kind for long-term relationships. But that set of people does not include everybody who has had multiple sex partners, even though all of those people are probably promiscuous. You're being silly.
It's much wiser to spend your time creating boundaries about how you deserve to be treated, not about what somebody's past is. Get out. Meet people. Date. Have sex if that is what comes natural to you. If you don't feel like it, don't have sex. But don't go berserko over past sex partners. You'll never compare up to the best of 20 other guys -- and if you think you have to, you don't understand what having a relationship means.