The Hospital Bicycle
Let's just say that Mark rightfully earned his reputation as a man-whore. Here's a rundown of some (but not all) of the women Mark has shacked up with.
Addison: Mark was one of the key factors in Derek and Addison getting divorced. The two of them even conceived a child, but it was terminated before Addison's move to Seattle.
Callie: Mark had a one-night stand with Callie which resulted in the birth of their daughter, Sofia.
Lexie: The love of Mark's life. They went through a lot together which included Mark committing Lexie after her nervous breakdown. Ultimately, they didn't have enough time to get back together before Lexie's untimely death.