The male readers here should understand that there’s a difference between a woman who has had a lot of sex partners, and a woman who is unloyal.
Having had a lot of sex partners doesn’t mean she cheated on anyone she was in a relationship with. It’s possible that each partner was devoted a segment of her life that she was dedicated to trying to make it work with them, even if that segment of her life was only a few weeks long. And if the latter is true, look for signs that she’s making progress and effort towards longer and more engaging relationships with men.
If disloyalty is what you’re on the lookout for, evaluate that based on the attention and care she pays to you over time, not on the stripped-down number of men she’s had sex with. As a woman in her late 20s who’s slept with over 20 men and had only 1 one-night-stand in her life, I think it’s important to make this distinction.
Athol:  Imagine for a moment that instead of that being your sexual history, that was your employment history, and your potential future husband was in fact a potential employer looking for someone to fill an absolutely critical position on a permanent basis. Should you fail in that position, not only would you lose your job, but the company would probably come close to failing as well.
Would you take at all seriously any applicant for a permanent position that had a resume showing 20+ different employers over the prior decade? Or would you simply see that applicant as far too great of a risk?
Ending the employment metaphor and returning it back to sex… you’ve not displayed any ability whatsoever to get a relationship past the first rush of dopamine / infatuation and into anything more stable. Either you’ve dumped the guy or he’s dumped you within what appears to be an average six month period over twenty times. Why would a man be willing to stake half of everything he has on being the 21st guy, in the hope that you do things completely differently this time around?
There’s far more to loyalty than simply not having sex with other men behind your husbands back. There’s the loyalty of being able to work through the repeated dips in the relationship rather than bailing every time one happens.  The best predictor of future behavior is past behavior and you appear to have a persistent pattern of relationship failure.
Jennifer and I have a great marriage, but we’ve also had dozens of bad days with each other. We’ve also had a tiny handful of mortifyingly awful days together. But we roll with the punches, work through those days and bounce back happier and stronger. Ask any long term couple if they’ve had really shitty days with each other and they will all say that it happened to them too. Unfortunately for you, you’re effectively telling us that when those shitty days happen to your relationships, the relationship ends.
So again… why should a guy stake half his stuff on you? Why should a guy stake his house on you?  Why should a guy stake access to his future children on you? Why should a guy stake a huge chunk of his future happiness on you?
Your fundamental flaw is thinking that what men look for in a short term relationship, is the same thing they look for in a wife. The Hookup Marketplace and the Marriage Marketplace have a few different rules. To be quite frank, as soon as you give it up to a guy without extracting any level of commitment, your Hookup Marketplace value rises, but your Marriage Marketplace level drops.
And not for nothing, once you figure the math out for even a few degrees of separation of sex partners, the total number of the sexual pool you’ve been swimming in is quite sizable if you’ve had 20+ partners. So obviously you’ve been exposed to STDs at some point, and it’s more a question of how perfect your protection against them was at the time. STDs can affect a woman’s fertility quite tragically, so it’s a common sense thing to be concerned about any STD history you have had and whether or not your fertility has been affected as a result.  As you can imagine, that would be need to know information before anyone should sign you up as their wife and future mother of their children. Getting you pregnant shouldn’t be as hard as shooting proton torpedos into a two meter wide thermal exhaust port on the Death Star.
And please don’t misunderstand me here, I’m not calling you a slut or immoral or anything similar. I do realize there are different relationship strategies you can choose in order to find happiness and what you do is up to you. There’s a very large advice industry encouraging women to pursue exactly the strategy you have chosen, so I assume they will be able to explain how to carry that strategy through to a happy ending better than I can.

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