The norm among the upper classes at least up to the late 1800s was arranged marriages. These were unions arranged for economical and political reasons rather than love, and while some couples did find love in these relationships, it was an open secret that you sired children and arranged your affairs with your spouse and found love with a mistress or a lover.

The counter-culture against the French revolution (which allowed divorce, among many other things) and the emerging burgeoise rejected both the debauchery of the older upper classes and the sexual liberty of the revolution. The romantic idea of marriage for love as a standard (rather than a sad tale of elopement and perhaps death) came to be during this age and the new burgeoise took to it to distance itself from both the old elite and the revolutionary ideas.

sex was terribly taboo, no?

No, why do you even think it was? It always ebbed and flowed.

Stefan Zweig in The World Of Yesterday described Vienna around 1900 and the extremely tight and strict sexual mores. But the point is that it was typical of that particular period and not all periods ever before the todays age. It wasn't even because of religion - that was already on the way out. It was actually a form of Englightenment rationalistic values, according to Zweig - that man must be above all rational, and they feared that "animal instincts" endanger this rationality. (Thus even the legs of tables had to be covered and women did not utter words like "pants" but rather said to men "you have a wine stain on the... lower parts of your garment.")

I can't give an overview of exactly how sexual mores changed in various periods, but there was an ebb and flow.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, you may PM /u/caffarelli to have your question considered for an upcoming Tuesday Trivia thread.