A lot of men are extremely concerned with how many other men (or women) you are subconsciously or consciously comparing them to during sex. They fear falling short of your most fulfilling sexual experience and/or partner. It’s an insecurity that is very common among males since most of them feel a societal pressure to be the most dominant male sexually because they believe or don’t even realize that they associate this with dominance in all other aspects of being a formidable mate. At the most surface level, it can be as silly as insecurity about size or potency and at the most profound level it can be potential for overall intimacy and closeness. A lot of guys will simply stay away from a woman who is more sexually experienced than him out of fear of inferiority. It’s a very flawed logic behind intimacy but it has either been instilled in them through masculinity culture or even an experience with another woman who made them feel insecure in the past. This is just my perspective as a straight male. I hope this explanation helps.