This is INCREDIBLY offensive. I am not a cow and I don't have milk, and a relationship is about more than sex. Believe it or not, there are plenty of men who do NOT base their decision to get married on whether their partner will have sex with them. And any man (or woman, for that matter) who would say "I don't need to get married, I'm already getting laid and I don't want anything else" is worthless. Did you miss the point of the women's rights movement being that we don't have to use our body parts to manipulate men into taking care of us anymore, because we can take care of ourselves? That women have more to offer a relationship than a hole and a heartbeat? It's real easy to sit on the other side of your keyboard and call someone a slut because she dares to have a relationship that you personally disapprove of, but it really doesn't make you look superior, it just makes you look petty and sexist.
And no, I don't think everyone who gets into a relationship with a separated-but-still-married person is horrible and deserves to be ridiculed, nor do I think they are all being taken advantage of by their boy/girlfriend. Some of them, maybe, but the blanket condemnation and insults are uncalled for.
I don't necessarily think this OP is the poster child for morality either, but every time I see this archaic bit of rhetoric thrown around, it pushes my buttons.