Think of it from a purely biological perspective for a bit. You are a caveman looking to impregnate a woman to raise a big family and continue your bloodline.
Which woman do you chose? The cave woman that has had 100 different mens DNA inside of her or the cave woman that has had much less? You want to chose the woman that allows your sperm the greatest chance of survival and you want the one who will not pollute your future generation.

You want to bond with your cave woman to the highest level, spiritually, mentally and physically. The cave woman that has been with 50 cave men has lost a lot of her pair bonding ability, she has more male DNA in her body and on a purely instinctive level, your biological warning sense tells you to run.
Just as the cave woman seeks the strongest male to provide and protect her and the future children, the cave man looks for the most fertile and untouched.
The above was what a guy told me when I showed him a similar thread about number of sexual partners. He said that he was repulsed on a level out of his control so thought it came from the cave man times haha.

Personally, a womans number of partners wasn't anything at the fore front of my mind but after I met a woman that accidentally let the cat out of the bag and tripped herself up with her own lies, I found the fact an instant and pure turn off. It was to such an extent I didn't understand myself at first.

Women label it as insecurity but I would argue that anyone who hides their past or is afraid of the truth and afraid of being open were the insecure ones. The only reason why women label men as insecure is because deep down, that's all they got as a defense or a possible way to attempt to escape the outcome, shaming in the hopes the guy will buy into it and let it go.

I see the labeling someone as insecure for not celebrating your high number of partners as an attempt to shame them into thinking that it is their problem. If you go back to my little cave man story, it would be equivalent to the promiscuous cave woman labeling the alpha cave man as too weak to mate with her in the hopes he had a fragile enough ego to make the mistake of mating with a woman that gave his sperm less chance of survival.

Just a story based on what my buddy said, not to be taken too literally but you get the idea. Oh by the way, my buddy rejects promiscuous women but he is one of the most secure guys Ive ever known so good luck with that insecurity label.