There seems to be a big factor, more so amongst men, that if the girl has done it but you haven't then it becomes an issue. It is with me anyway, something to do with ticking boxes.
For example, when I was younger I almost always wanted the girl to have slept with less people than me, the actual figure did not matter as long as it was less and this still happens now though not to the same extent...that then tends to lead to judgements.