C, I'm not gay even though I want her to play with my ass. Sounds silly I know but the standard thought these days if you are male is, if u want it in the butt you're gay.
Fairly simply really. Tell her why you want it (psychological aspects as well as what it does to you physically), and that you are not aroused by the male body so thus not gay.

Perhaps treat her too a romantic night by cooking for her. Have the house clean, draw her a bath maybe and just pamper her. Retire to the bed room, give her a back rub, and go down on her. Make sure she enjoys herself. Make it about her. Open up a bit and let her know you want to serve her like this.

If you go with watching a porn with a girl using a strapon, probably going with one that is more romantic and less hard core domme will go over better initially.