It's so natural, eating ass, nothing gross at all unless that's part of your fetishization of the act, in which case calling it gross, dirty and filthy is verbal foreplay.
Nonsense. She is totally into eating his ass and quite possibly has a fetish for how that area smells, as she keeps rubbing her face into his ass and balls. I truly appreciate a woman who does that, I've slept with women who are into it and fetishize the smell of a man's balls and ass, but coaxing them into doing it means putting their fear of being ridiculed to rest.