Pregnancy of Revenge question

There is something that I don't understand in most of the novels that I so far read, why the virginity of most heroines must be not well acknowledged by the heroes?? why there is almost no mention of the blood that should and must by seen at the first time of intimate encounter?
as far as i know this has to happen to all females on their first! so why the hero is most times confused with this fact a only she is being tight!! I do not like that .
I hope I can find an answer to my question!