I had my first sexual experiences with my first long-term partner. We were together for almost seven years. And the sex wasn't that great then, to put it in polite terms, and it's even worse when I look back at it now. Your, let's say, romantic idea of discovering sex together completely forgets that people have different sex drives, needs and different kinks that they want to have satisfied to live happily. So imagine I married my first long-term relation - boy would I be miserable today.
Making sex dependant on the institution of marriage in this day and age will only have one result, that being to make marriage even more of a disposable commodity than it already is.