Cuckold fantasy

I've always had this fantasy of watching my girlfriend getting fucked by an older guy. I often find myself wanking off when I think about it. My girlfriend has no idea and would probably go mental if she knew I was even thinking about it. My other half has an amazing figure, her tits are are quite small but when she's riding my cock her nipples stand out like bullets. Her best feature I have to say is her gorgeous arse it's really curvy and grabbable. If she's wearing anything tight men just can't help but stare. She looks amazing in a tiny g string and she has really big cunt lips.
It all starts when an old stranger comes to our house one afternoon my gf is sitting next to him in a really short skirt with stocking tops showing. After chatting and flirting with him while I watch, she stands and slowly strips for him down to the tiniest black g string and bra and invites him to stand and play with her arse and tits. He undoes her bra and releases her pert tits, her nipples so erect. The scene is so hot I'm already wanking hard and furious. He strips naked and reveals his massive rock hard cock and she begins wanking him hard and fast he looks as though he could come immediately. She leads him upstairs and on to our bed, pulls her knickers to one side and rides his cock like a woman possessed. All this while I'm sitting there watching and wanking. The old guy is having trouble holding back and as his now quivering cock pulls out of my gf's juicy cunt he squirts what seems like a pint of hot spunk all over her arse. This tips me over the edge too and I shoot my load everywhere