I can understand from a more conservative standpoint. I think a high partner count is associated with risk-taking, independence and a certain mindset (sex over emotion). All three of these characteristics are incompatible with what more traditional relationships require of a girl.

The partner count is demoralizing. Demand and supply - it feels very nice driving a rare car (Lamborghini, let's say) than it feels driving a Fiesta. You're there thinking like 'man, she put out so much, she must have low standards'. And it gets to your head. Is she settling for you? People like to say "after 11 people, she chose to stay with you". Yeah sure, but I feel like why she chose to boyfriend you is a better question. She's seen so much dick that it's statistically unlikely that yours is the best she's had. So chances are that she decided to 'settle' for you because you support her emotionally and treat her good, not because you're good at sex.

It makes you insecure thinking stuff like this, stuff like "yeah I'm probably not giving her the best sex" and "she's probably had dick way better than mine, but she's only with me because I treat her better than the other guys". Some would then say that sex isn't everything, and that it's great that you treat her better than previous partners. Well, sure, but in the broader scheme of things, doesn't this make me a less valuable partner? It's very subjective, but I feel it does. I feel like you can put individuals in a hierarchy (dating wise). The guys that did not want to get in a relationship with this girl feel they can do better. So by getting in a relationship with the girl, you're implicitly admitting that you're of lesser value than those who have broken up and moved on from her.

I feel like people try to brush partner count off like it's nothing, and try to rationalize it in a twisted way. It's not necessarily easy to accept a partner's slutty past. And frankly, I'll play the devil's advocate - if there are two identical twin girls, except one has a partner count of 20 and the other only 3, the one with the less partners will be far more suitable for a relationship and will have far more value on the relationship market.

I've dated numerous girls with more partners than me. So I can guarantee you that this lingering feeling of you being the one she 'settles' for does not go away. After you get some reassurance from /r/sex, you'll feel good and you'll think you're over it. But that's far from the truth, those feelings will keep coming back, in a week, in a month, in an year, in twenty years. It's something you should keep in mind. You really need to ask yourself whether you can get over her past.

And honestly, I don't think you currently could. The reason I'm able to date girls with a lot of previous partners is because I've had a lot of partners myself, which gives me validation. And second, you kind of get callussed over time. Good luck finding a girl in her 25-30 that has a partner count less than 10. I'd say in my experience, girls aged 25-30 have a partner count of roughly 20.