I want my gf to be fucked by someone else

So me and my gf have been dating for about a year. We have great sex. She is super curvy and I’m pretty athletic build. She really enjoys our sex. Let me first state that my dick is about 8 in long and 5.5 in thick. Not too bad. She loves it. She have been role playing about her taking another cock from another guy with her dildo. She gets so turned on. I really think she would enjoy another big cock like mine if not bigger. I want to watch her being even more spread open than I can and tossed around. I want to hold her legs while someone thrust into her. Seeing her getting pounded turns me on so much. So question is, how should I ask her if she would and would guys be really up to pound my gf while I watch and or join?