Fiancee and my Dad.
One of the sexiest fantasies I have is to watch my dad have nasty s e x with my the woman I'll marry. During my younger years, I walked into my dad having sex with my stepmom and was impressed with her ass, but also noticed was bigger for some reason.
Well, during a night of passionate sex with my fiancee, we were sharing stories and I told her about my dad and stepmom. She wouldn't stop asking about his size, which I estimate to be around 9 inches and pretty thick (I'm 6). I noticed sex was 100% better once I told her about dad. Dad is only 56 (I'm 36) and, under the right circumstances, I know my fiancee would fuck him in a heartbeat.
I don't know where this fantasy originates but it's there and I find it incredibly arousing when I masturbate. Instead of my stepmom, I imagine my fiancee's big ass bouncing on his fat cock until it explodes inside her pussy.
BTW, my fiancee finds my dad really sexy and cute, so this adds to the fantasy.