Feminists respond by saying that our patriarchal society has brainwashed men and women to think women who have slept with "too many" men are sluts and lesser beings, unworthy of serious dating. This way, men can control female sexuality and can have yet another way to dominate the opposite sex. I'll admit that social conditioning definitely could play a factor but I do not think that is the main reason most men are so concerned. I think there is a much deeper male psychological issue but it's often too painful for men to talk about openly. Since I have somewhat of a defective mental filter, I will try to speak on their behalf.
The reason is actually very simple: men are insecure. This is true of every man in the world, even the ones who are rich, handsome, smart, and famous. Men are just as insecure as women are but we are conditioned and wired to not show it as often. Men know that women are attracted to confident, high-status alpha males so they try to minimize whatever signs of insecurity of which they are aware. The problem is, our egos can be quite delicate and can be hurt by the external world.
Like women, men receive feedback from the world about who they are that form their images of themselves. For example, you notice that whenever you sing, people often compliment your voice and musicality. These compliments will enter into both your conscious and subconscious to shape your self-image. You think to yourself, "I must be a good singer! I must have some degree of musical talent."
The same thing is true for dating. Just like women, men want to feel special. There is a certain amount of intimacy in sex. This is true even if you do it for pay. Unlike other animals, we don't regularly expose our naked bodies to strangers in public. Sex is also what keeps our species alive. Biologically speaking, love evolved to potentially keep us together long enough so that we can raise our children well. Thus, there is some link between sex and love in our species. After all, we are all part of a series of vital sexual episodes that have been unbroken for billions of years. Therefore, if a man finds out a woman has slept with more than a certain number of men (whatever that arbitrary number may be), he will question how much he really means to her. This will also trigger other insecure thoughts such as "maybe I won't be able to satisfy her sexually because she has had better", "maybe she'll leave me for someone else since she doesn't value men", or "maybe I'm a man who can only get women who sleep around."
This discomfort is very real and very painful for many men. In fact, this destructive thinking can very well ruin a potentially very happy and fulfilling relationship. She can be great in every single way but just the fact that he thinks she slept with too many men can be a barrier to connect at a deeper level.
Look, women are sexual beings. If they weren't, we wouldn't exist. Even an introverted church-going woman will have intense sexual fantasies most men wouldn't even imagine; given certain circumstances in life, she may readily indulge in an outrageous sexual encounter in the heat of the moment. We live in a (partially) free culture where individuals can pursue their own happiness as long as they don't harm others. We even have a responsibility to reinforce this and affirm (and respect) everyone's liberties.
Men, if you really like a woman and sincerely care about her and she truly feels the same way about you, then you should overcome your negative thoughts brought about by your insecurities and try to forget about the men who are probably not even in her life anymore. In the grand scheme of things, what is most important to you? Odds are that you want to be with a woman with whom you can have a happy relationship and have a family. Most women want that as well, even the ones who have slept around in the past. Many women sleep with various men because they just want to feel appreciated by a man but those men end up leaving them. Why can't you be that man who will love her and support her?
All of this is easier said than done. But the truth is, with time, a man can get over it. Plus, there will be plenty of other things to worry and fight about later which will pale in comparison to this issue (e.g. living together, managing finances, finding employment, getting her pregnant, trying to get along with in-laws who don't really like you).
There is one more major takeaway: knowing yourself. Ultimately, all of this will bother you less if you know who you are and what you want. Other people's opinions and actions won't phase you as much. If someone calls you "stupid", that insult slides right off of you because you are confident in your own intelligence. If someone calls you "ugly", you know that is one person's opinion and that person probably said it out of anger and not rational thought; you know that there are people who find you beautiful and their opinions would matter more anyway since they probably know you better than some irate stranger on the street. And even if your partner has an extensive sexual history, you know you are awesome and are able to value him or her in a way others in the past have failed to do.
I hope this clarified some things for men and women. If you are much older than I am, then you are probably familiar with all of the above. If you are younger, then I hope this was somewhat helpful even though you are going to make the same mistakes your elders made and warned you about. I guess you have to live life to really learn from it.
