Not a big impact. I'm way too rusty for specifics, but some choices give you various demon bits; you get a dick to screw her with if you don't already have one, and if she fucks you it adjusts that part's values (ie: butt fuck makes your butt more accommodating). Fucking her probably gives you a demon dick or makes you bigger.
All demon bits are removed shortly after, and by "shortly", I mean they only last that one sex scene. All breast/orifice/size modifications stay. Not sure if the new herm dick gets removed or just de-demonized.
Basically, you're in the prologue, and nothing has any permanent plot ramifications. However, when you wake up, I suggest you accept the help offered you. She's adorable after all.