I used to think there was nothing wrong with it until my roommate actually started dating a girl who was separated but not divorced yet. Aside from the fact that she was an insufferable wordIcan'tusehere, the biggest problem came from the fact that she was ready to end her marriage, however her husband, an army man in Iraq, was not ready to end it. And the whole time it went on I warned him if an angry army man came to break down our door because he was after the guy who was sleeping with his wife, I would simply toss him (my roommate) to the husband and say, "Here ya go, please don't shoot me." And the more it dragged on the more you could tell she didn't love him, she was using him to make her husband she kept claiming she was going to leave angry. She kept saying she was going to the courhouse to get the papers started, but she never did. And wouldn't you know it, as soon as her husband came back from Iraq, she decided, "Oh wait, I still love him. Bye!" and moved to another state with him to "start over".
Moral of the story, if you must date someone who has been married, wait till they're actually divorced. It's like the stereotypical stories you see in the movies where someone has an affair with a person who keeps promising, "Oh I'll leave him/her soon and then we will be free to be together!" Never ever believe that line. Whether you have religious issues with it or not, I say dating someone who is only separated but not divorced is a bad bad bad idea.