Oh, and it’s “a big deal” because most married people understand that sex is a sacred act. We understand the risks WRT disease, pregnancies, and unstable mistresses/boyfriends who become obsessive — sometimes with fatal results for the rest of the family.
You wouldn’t understand because you’ve obviously never felt the deep emotional bond with another person that would make you want to spend the rest of your life and possibly have children with that person.
For single people (especially you and Brian, apparently), perhaps it’s not a big deal if someone sleeps around on you, but don’t assume that married people should share your beliefs. Our realities are far different from yours.
BTW, do you (honestly) encourage all of your girlfriends to sleep around on you? Does it honestly not bother you?