There is this little concept called sexual compatibility.
It basically means that sex and sexual activity can be better or worse depending on the person you're doing it with, and since sex and physical attraction is an important component of a relationship, people need to be physically intimate and have sex (ideally before marriage) in order to see whether or not they are sexually compatible.
People in relationships without sexual and intimate compatibility are likely to be unhappy and their marriages are more likely to end in a number of miserable ways; the divorce being the most obvious example. So, to summarize, people are biologically driven and motivated to have sex, sexual compatibility is an important part of relationships and therefore, no mater your beliefs, sex before marriage is not only healthy, but highly necessary.