It does not phase me in the least. Whatever, wherever, however. Whatever he needs, or wants is fine with me. I am game.
At first, with any new thing we try, regardless of size, I always ask, "Is this ok?, How does that feel?, Do we need more lube?, Faster?, Slower?, Deeper?" I am unsure of new things, so I tend to talk a lot when we introduce something new. I want to ensure I am delivering pleasure and nothing else. The reassurance builds my confidence.
And no, I don't think butt play or the size of whatever you like in your butt, has any indication of a tenancy toward gay-ness. I like butt play too, that doesn't make me a lesbian, so why would it make my husband gay?
Enjoying butt play has nothing to do with sexual preference.
If she is new to your toying experiences though, I can see where she might be taken aback and I can understand her reluctance. I agree with what others have said, start small and un-intimidating for her sake.