Your mother-in-law is wrong obviously. Have you told your girlfriend yet? It is obvious that your girlfriend and her mother have talked about your issues behind your back. What happened to you as a kid wasn't your fault, nor are the unresolved effects of that victimization. Your girlfriend is throwing-up because she drank too much. And it seems that everyone is blaming you because you are not responding in a manner familiar to them.
I think that you have two choices. Either you are going to have a cursory yet frank discussion of your issues with your girlfriend hoping to get some understanding, or you are going to have to try to let yourself go just a little bit. The first strategy would hopefully buy you some time. You might also talk about the hurt that you feel when you get it from her mom too. This strategy does not work 100% of the time, and it could cost you your relationship with your girlfriend too.
The 2nd strategy would be to try to let yourself go, at least a little bit. Before I started into my recovery the only two-party sex that I ever had was when I was drunk or stoned and when the young lady (or young man) wanted me. I found initiation of sex to be just impossible. My first wife threw herself at me after a long night at the bar when we were both drunk and stoned. By then I had several short or one-night relationships after hard drinking or using drugs, always when the girl wanted it. While my first wife and I had a decent relationship, I never told her, and lived in constant fear that she would find out. When she did find out through an argument with my mother, she divorced me, at a really bad time for me.
So I guess that I would opt (looking back) for some kind of sharing before your relationship is totally on the rocks. I wouldn't go into any details. You could reiterate that you don't enjoy having sex while you are drunk and you should also let her know how it makes you feel when you hear it from her mother. Maybe you could try to let yourself go just a little bit and try to share just a little bit when you and her are intimate. Maybe you should try to get a copy of Mic Hunter's book ABUSED BOYS and leave it out where she might see it. That might be one way to broach the subject. His book is sold through our online bookstore off of the homepage.
If your relationship is totally on the rocks and your girlfriend seems to be upset about your perceived inadequacies, I might wait until she has calmed-down and you are on a better footing with her. For tonight just try to soothe her and try to get some sleep. Better be ready with some Excedrin and some TLC for tomorrow if she wakes-up hung-over.
Remember, my man, what happened in your past isn't your fault. You are beginning to deal with what happened and its effects and you are making progress. You may need to try to buy some time and having your girlfriend's support would be a big help. We will be here if you need to talk more. Hope that you can patch it up with your girlfriend and hope that you have a good Christmas with her.
"We stay here, we die here. We've got to keep moving". Trucker Mark