You don't need to get dressed like a girl to get fucked in the ass, I can get ass fucked any time I want, but sometimes I do dress the girl way just for kicks.
I had sex with my gf while wearing her thong; not sure if she knew it or not.
I was wearing a thong one night and my buddy fucked me, he didn't even take the thong off, just pulled it aside.
You don't have to wear her thong, they have thongs for men, they feel nice, the way that they stroke your ass and hold your balls thight.
Yes that's hot. That's how I started crossdressing and sex with men. You enjoy it so much because it is lowering your inhabitions and you are doing what you really like. When you do get your ass fucked and you will try to make it last so you can really explore it deeper. Some times people feel too ashamed to do what they really want. Take it slow and enjoy. P.S. Both you and your GF need to be happy so work on her needs too. I know how enjoyable it all is and easy to get overwelmed with emotions.