So who's to blame?

My girlfriend's rational for seeing someone who has a girlfriend is "that's him cheating".  She said he's not married to her, nor does he live with her so he's open to see who he wants and as far as she can see that's on him.  That may be true it's on him, I agree he's the one in a relationship.  The other woman doesn't owe the girl anything nor an explanation for that matter.  But then my question becomes why would you continue to get involved with someone you know is taken.  How does that benefit you?  Outside of the sexual gratification in which in your case is not just a sexual relationship.  You do have feelings for him and for the most part you do care for him.  So why would you play with your emotions like that.  You say for him to cheat has something to do on her end or the relationship is falling apart.  So you think because he hasn't married her yet or they are not living together that you're going to discount their relationship.  As far as I can see to say he has a girlfriend period, trumps most reasons as to why he's not getting involved with you.  Most guys would lie to say their single so for him to say he at least has a girlfriend is a major red flag that he's not looking for anything more.  But of course you can't see that, you see since he's not married nor live with her, you think it's not serious.  You think there may be a possibility for you to be included in his life, once he gets to know you, right?  That may be possible, but why would you want to be someone who's involved?  Why would you be ok just to be his appetizer when he has a full meal to come home to, that's waiting for him.  Why would you want to be his back up plan when he made plans with his girlfriend to wine and dine her and treat her his queen and then run to you when he's bored.  Are you ok with the half fast relationship?  So outside of the sexual relationship that you only have what makes you think he would treat you differently?  That's the problem you don't think!  You're thinking about you now and the selfishness you have for yourself.  But you know who's really selfish is HIM!  He's having his cake and eating it and everyone involved with him will get hurt in the end. He's not thinking either.  Of course not and the rational behind it all is ........what again?