No one has to do any such thing. The generally accepted morality that we've abandoned is a loss to society as a whole, especially to the out-of-wedlock children these casual relationships produce which is almost as bad as the soul-deadening fact of a million-plus abortions performed in American every year, most on white, middle-class women who are not married.
Serial sexual 'relationships' between unmarried young adults are ultimately depressing and the fact that this is now considered 'normal' does nothing for the men and women involved who end up wondering why (eventual) marriage is no big deal and often divorce in a few years as the marriage was just a nother 'relationship' and held little meaning after all the sexual affairs that came before it. It's sad and although there are always exceptions, this tends to be quite common in today's 'anything goes' society where sex between unmarried young adults is taken for granted, if not expected.
Forty-plus years ago young adult men and women still had casual relationships, sometimes many before marriage, but sex was not automatic or expected, although of course some did engage in it. There was no perceived need to 'try it out' much less play house and pretend it was a committed relationship - except that either one could flee at the first sign of a problem, boredom or a better sexual partner.
We've lost something important as a society in the name of, what else? Freedom. Well, we're free alright. Free to be promiscuous and call it a 'relationship'. Free to have a million abortions a year. Free to contract STD's by the millions and many women are free to face thirty-five unmarried and feeling used with all illusions shattered and possibly a fatherless child in tow.
No, no one is forced to remain chaste before marriage (and it was never a law in America that anyone had to) but with the permissiveness and frat-boy mentality we've adopted when it comes to sex between unmarrieds and the meaning of 'relationships' being one of sex and convenience and little more, we've lost a lot, even if it was voluntary.