[F4M] We can do anal, daddy, just please don't tell anyone it's SO embarrassing.
I like imagining that I just occasionally help you out but mom otherwise has you covered (if you're a daddy, uncle is fine too!). In my fantasy, I'm your adult daughter living at home while I attend community college. Like lots of families, you and mom have agreed that it's just helpful to have me give you sexual favors; you and mom have a great sex life, of course, but sometimes she's not around or you just enjoy a little variety.
One favor you ask me for now and then is back door. I find it a little mortifying, for very good reasons! It's uncomfortable, for one, because it's so tight, but that's almost the least of my problems. I just can't help but always thinking of it as a dirty place no matter how clean I get, and it's the last place I want anyone looking at or touching or sticking their cock inside. I'm always a little terrified that I'll get poo on you or make you smelly, even though that's never happened and the prospect doesn't seem to bother you at all. And it seems so pervy, I'd just die if mom or one of my friends ever found out that I'd done it with my dad. I KNOW lots of girls do it with their dads, but it never would have crossed my mind if you hadn't asked.
So I do it for you now and then - I don't really like it, but I like making you happy. Mom's out of town for the weekend to visit her mom, and of course I'll give you sex while she's gone. I figure you'll ask for back door while she's away. And you'll probably talk me into it since I'd rather do it with her gone than when she's around.