The wife need to talk to the husband and discuss the true reason wy she doesnt enjoy sex :

Possible reason
a) The Husband makes no effort to please the wife during sex and only serves to please himself.
>> This could be an unexperienced husband and he might not know that he is not making his wife happy this way.

b) The Wife is not comfortable with sex before marriage.

c) Their relationship is already facing problems before hand and they thought that marriage/sex will solve it for them and make them closer.
The actual result falls short of expectations and so this issue crops up.

talk to a counsellor they can help you.Marriage is not a date or a relationship, marriage is serious and it is meant for people who want to spend their life together thru thick n thin.