Dear Abby: My daughters are attractive young women, both doing well in their professional careers. "Melanie," who is 27, is married to "Sam," an extremely attractive and successful man.

My 30-year-old daughter, "Alicia," has been divorced for a year. Her marriage failed two years ago because she and her husband had an appetite for sex outside their marriage. While I was disturbed about that, I was horrified to learn that Melanie allows her sister to occasionally have sex with Sam.

Melanie's argument is that Sam is less likely to cheat given this situation. When I asked her and Sam about it, he said it wasn't his idea. My current husband says any man who would refuse this "set-up" would be nuts. Alicia claims she "doesn't have time" to date right now, and after she finishes her MBA, she'll seek out a more normal relationship.