Many times on this forum I have posted the following (clean version):
Gentlemen: Only ever “do it” with your wife.
Ladies: Only ever “do it” with your husband.
It seems, for this thread, I have to offer this, as well:
Gentlemen: Be the sort of man that your wife would want to “do it” with.
Ladies: Be the sort of woman that your husband would want to “do it” with.
Desirability is a necessary adjunct to fidelity.
That works.