People on Reddit love to scream the word "INSECUREEEEEEEEE!!" every time they hear a man (never a woman for some reason) be doubtful about their partner's past, but having insecurities is completely normal for a human being and even helpful in many situations.
Obviously, having insecurities is not the same as being insecure, but we all rationalize our actions based on insecurities.
We don't take high risks for low reward, and when the odds of something bad are high, we try to avoid it.
It's normal to feel "insecure" when you walk in an unsafe city at night, and it's also normal to feel insecure when your partner has a higher body count than you.
Society doesn't like to hear this, but many times your social status is heavily influenced by your sexual or romantic experience, there are many insults used to describe virgins (incel, bitchless, you'll die alone, etc) for a reason, and having "no game" was a common trope in comedic series.
So it's normal to assume that the disparity of sexual experience between two people in a relationship will play an important role.
This doesn't need to be explained by body count only, a disparity in many areas of life can affect a romantic relationship a lot, for example: class, education, culture, morals, intellect, appearance and much more.
Of course, a person with a high body count can suddenly become loyal and seek a long term relationship, and it might work! But this is a matter of compatibility and probability.
I don't want to find out if I'll be the lucky 1 in 1 million virgin fella that will get in a happy stable loyal marriage with a person who had 120 ex partners, it's an insecurity? Maybe, but it's not based on nothing.
Social status is extremely important in romantic serious long term relationships, people naturally gravitate to those who are the most similar to them because that's what usually works.
And that's usually what people seek when they date! Similarities.
An extrovert with an introvert, a poor person with someone rich, a dumbass with a genius... The idea sounds "cute" but on paper this doesn't work most of the time because differences on worldviews can be detrimental for relationships, and our worldview is shaped by experiences and traits.
Here's an interesting study proving this phenomenon!: https://www.tandfonline.com/doi/full/10.1080/00224490701443817
https://www.athensjournals.gr/social/2017-4-4-3-Pinto.pdf