Nope, not even if they were in the process of a divorce.
My ex boyfriend cheated on me, so I know what it's like to be on the receiving end of things. It hurt enough without a marriage, children and a mortgage involved. To date/sleep with a married person is wrong on so many levels, not only are you contributing to the break down of a marriage, but you are putting yourself at the risk of some serious backlash.
I can't understand people that do it.
