I have not had the conversation with my current GF.. but my ex loved the idea. Once in a moment of honesty I told her that I used to fool around with guys when I was 15 - 18 at boarding school. She wanted to hear all of the details and I could see that it got her totally hot. She asked if I would do it again and I said yes.. for the right guy. She was really keen to set this up. I was keen on having a gay guy as I perceived this as "safer".. I guess less of a threat and more attention on me or something. maybe also wanted someone who was sure about themselves

Having said that I was also keen to see her blow another guy and get fucked while I watched. After that we shared gay porn but we never managed to set up the threesome.
I then moved away, but we kept in touch. She said she found the perfect guy at her gym. He was a personal trainer and gay, but super hot (she described him and he was just my type - tanned, toned, hairless, cute and boyish). We fantasised about it but I haven't seen her in a couple of years and we are both seeing other people now.
A few other girls I have spoken to like the idea of guys touching each other and oral.. particularly the fantasy of young guys experimenting, comparing cocks, wanking together etc. They enjoy that and when I mention that I went to an all guys boarding school, sometimes they make a little joke about the boys all wanking together.