She did! She met someone who she believed was unavailable, which sounds like exactly what she wanted...that's the problem!
In this situation, [Miss D] truly has no one to blame but herself!
Accept the Consequences of Your Actions
Whenever you get into a relationship with a married man, you have to accept the consequences of what might happen if you are found out. Whether or not you are the reason his wife left him, now you have to face the consequences of what you've done. Whether or not she knows about you, HE did...and does.
He probably fully believed that you would be there for him when she left...perhaps it even gave him a different perspective on her leaving. Just because you assume he is unavailable by way of marriage, doesn't mean he thought that is what you were interested in.
You Led Him On
So now he wants more than you do? Sounds like you were awfully comfortable with this man who couldn't give you a full relationship..as long as that is how it stayed. Now he expects more from you, that I'm assuming you have led him to believe he can expect.
Tell Him the Truth
First of all, you need to be straight with him...and straight with yourself. Most people who get involved with a married partner aren't truly looking for a solid relationship...he should realize that about you, and so should you. Don't string him along...let him go, tell him the truth so he can move on with his life and make some decisions.
You Could Be Emotionally Unavailable
As for yourself, why were you so attracted to a married man, anyway? Figure out inside yourself why you wanted someone unavailable...it probably means that emotionally, somewhere you are unavailable yourself...if you can find that and deal with it, you may have a chance at a better relationship down the road..with someone who is SINGLE.