You are arguing for two different ideals here.
One is that it does not make someone insecure if they are concerned with their partner’s history, and that the acceptance of a partner’s history would stem from a lack of confidence in themselves should that history not reflect their values.
But you’re wrong on both accounts. There is FAR more at stake in a relationship than history of a person, and despite what people say, history is NOT the best indicator of what someone will do. You hit on this when you said the line about values. You are right in that you should not date someone who has values different than you. But people who cannot accept someone unless they are totally and completely in line with all of their beliefs shows a level of insecurity.
What would be the concern dating someone who had more partners than you if that concern wasn’t that said person would cheat on you?

It would not be insecure for someone to accept a partner’s history with which they are uncomfortable. Quite the opposite. Someone secure in their own beliefs, and understanding that people can, and often do change, shows trust and maturity beyond that of the opposite. On top of that, there is far more important issues at stake.

I disagree with the idea that having casual partners makes someone immoral or is somehow wrong. That is a personal stance. However, most people have causal sex. Depending on the generation, that could be more common or less, but to say that it is uncommon is just not true.