Are we talking about premarital heterosexual sex or homosexual sex (where of course they aren't married to each other but either both or one is unmarried)? Heterosexual sex is hard to say because getting married wasn't that difficult because like u/sunagainstgold being married only required that two parties agree that they are married. There are divorces cases were one party claimed that their current marriage was invalid because they were already married. People were married and "unmarried" all the time, so though technically not premarital sex people would get married, have sex, and then go their separate ways. So who knows, it did of course happen. Now for homosexual sex, in Florence, it was basically accepted that young boys would have sex with other men. So homosexual premarital sex was rampant.

I recommend Constructing Medieval Sexuality by Karma Lochrie and Handbook of Medieval Sexuality by Vern L. Bullough and James Brundage