I'm sorry, but I don't think it's fair to say Marlena couldn't keep her pants on before (or after) she met John.
Marlena was happily married to Don Craig and only divorced him after he had an affair after their baby died. And this was after she worked to keep their marriage together.
Her next love was Roman, and she never strayed from him during their brief marriage. She was devastated when Stefano "killed" him.
She eventually got involved with the police chief, Richard Cates, who was a crooked cop and got killed. I wouldn't call this relationship scandalous or anything.
When John returned, she began to fall in love with the man who would be her soulmate. The only problem was, she feared he might be Stefano. When it turned out he was Roman, her attraction to him made sense. She brought Roman back into her life and was happy to be reunited with her great love. Then she "died."
When Marlena returned, she couldn't wait to be reunited with Roman -- the great love of her life and the father of her children. They were reunited, and life was good. But then it was revealed that Roman wasn't Roman because the old Roman was alive and being held by Stefano.
By this time, Isabella was pregnant by John, and Roman wanted his wife back. Marlena and John did the only thing they could do, which was put their feelings aside and return to Roman and Isabella. When Isabella died, John was grieving, and Roman was becoming bitter about Marlena's relationship with John as Roman, things got complicated.
Then Stella threw Marlena in a pit and left her down there to die. Stella changed her mind and decided to rescue Marlena before John blew up the warehouse over the pit, but Stella was killed by a car. John found Marlena but was unable to rescue her; instead, he ended up in the pit with her, under the debris from the explosion. By this time, Marlena was dirty, malnourished, and delirious. In their "last moments" together, she started having flashbacks of both men and honestly couldn't tell who John was -- John or Roman. They kissed. Not long after, they were rescued, and Roman took his wife home.
John couldn't let go of Marlena, and Marlena felt torn. After all, she had spent about as much time with John as Roman as she had with Roman as Roman. John knew Marlena was feeling torn and planned to leave Salem. Marlena wanted to stop him because she was concerned about him, and they ended up sleeping together. And then it happened once more.
Marlena truly called it quits with John and treated him the way she's telling Belle to treat Philip. When Stefano arrived at Belle's christening, Marlena had to reveal what she learned too late -- that John, not Roman, was Belle's father. Roman quickly threw in the towel and divorced Marlena, and lived to regret it. Marlena and John eventually found their way back to each other and lived happily ever after, with a few detours (Princess Gina, Alex North), until recently.
What was so tragic for Marlena was that she was having to choose, basically, between two Romans! That's NOT the same situation Belle, Shawn, and Philip are in.
Couldn't keep her pants on? I just don't think that's fair. (Of course, now that I think of it, this saintly woman was once possessed by the devil, too.)
Chip