Does it affect a relationship when a woman have higher body count than her male partner?

I have a genuine question and would like to hear different perspectives from both men and women.

I have a friend who had a fair number of casual relationships in the past. She's now looking for a serious, long-term relationship. Things usually go well at first, but when the conversation eventually turns to past relationships or sexual history, the guy often becomes distant or ends up ghosting her.

It made me wonder:

Is this because many men feel insecure or that their ego is affected when their partner has had more previous sexual partners than they have? Or is it mainly a result of societal norms and the different standards men and women are often held to?

If two people are compatible, honest with each other, and capable of building a healthy relationship, why should the number of previous sexual partners matter so much?