I think Tony trying to be all strait-laced and Zen while Steve gropes him and mutters dirty things is - mmmmm. Steve needs to spend more time being unutterably filthy and blowing Tony's... mind. What? A good sex life is an important part of a happy relationship. Steve just wants to keep their relationship healthy.

There doesn't need to be a fusion. :p It can be Hallowe'en, they're wearing costumes and playing, which has the added bonus that Steve doesn't need to have committed terrible Sith atrocities. If only someone was offering Hallowe'en drabbles that we might request such a thing!