There are several ways to do this: the first is to get married, the second to find a prostitute.
Once your married, you should go to Bowerstone Market and enter the bookstore. Buy the book that gives you the "Come back to my place" expression. Then, use it on your target, lead them to a bed, and choose what kind of sex you want to have.
Prostitutes can be found in Bowerstone Old Town, Westcliff (if you invest before the spire quest) and Bloodstone. Give them the "thumbs up" expression and let them follow you to a bed (in a house you own, or an inn.) Beware of STDs, however. Using a condom removes this threat. Also use the Y button to check the stats on women. any women with unchaste, randy, kinky are easy to get in the sack (bed)