Various cultures have different traditions and conventions concerning sexual relationship. In modern western culture, pre-marital sexual relations were considered taboo for women but acceptable for men among the middle and upper classes; this gender distinction was based on questions of pregnancy.
The advent of reliable female birth control has enabled a massive shift in sexual mores, in which pre-marital sexual encounters no longer necessarily result in pregnancy.
On the one hand, pre-marital sex still does carry the dangers of sexually transmitted diseases and pregancy if one does not use condoms during all sexual activity. It can also be psychologically damaging if the participants are not sufficiently emotionally mature.
On the other hand, sexual experience helps one distinguish sexuality from emotional commitment, and thus makes it less likely that one may end up in a bad marriage by confusing lust and love.