It is much more than just the physical act. Sex within a marriage is intended to be the ultimate expression of love. The emotional side of sex is the one that is seldom taught. If someone doesn't understand that even if you are lucky enough to avoid the physical hazards the emotional scars can be longer lasting. Sex within a marriage is intended to be the safe place for that to happen,
Never forget it was God that created sex and the context in which is to be used. In that context it is neither evil or secretive.
