The key to openly date a married man without guilt is that his wife should know.
Assess if the married man is matured enough to handle a non-monogamous relationship
If the marriage has evolved to the point where it can handle new people or the idea of an open relationship is not new to the couple, then you may have passed the first hurdle. Opening up a relationship to new intimate connections is a much-matured decision which could backfire if the person has an immature view of the situation.
It’s best not to tangle with a married man who is only looking for adventure because he’s bored with his wife. There’s bound to be drama if the wife finds out. And that always doesn’t end well for the mistress/side-chick.
But if he wants you as an addition to an already great relationship, then your presence is much more likely to be welcomed by his wife too.
Make sure you get his wife’s approval
Take that bold step and speak to his wife. You may uncover certain information which may save you time and an enormous amount of hurt. Unfortunately, some men would lie through their teeth to draw innocent girls to help them cheat on their spouse.
Verify his story for yourself to be sure the wife is onboard a newly open relationship. Find out if she is happy to share him with someone else, if not please bow out. It’s not worth the explosive emotions that are bound to follow.
Evaluate his marriage with his wife
Does he have a good marriage with his wife? Don’t let yourself be the Band-Aid for their marital problems. How he treats his wife will give you a fair idea of what you are getting yourself into.
The ideal situation is that you're becoming part of a mature and well-established relationship. Don’t get yourself into any kind of circus.
Consider his whole family especially his children
Think about how his relationship with you will affect his marriage and children in general. If you don't care then you're probably not mature enough to get into a relationship with a married man who has already established a life with someone else.