Forgive this slave for being obtuse, but how is rimming and fucking your ass with her tongue any safer than ATM?
How true that this is the point of the discussion. This is so similar to what parents warn their teenage girls against. (Those sneaky teenage boys that tell their girlfriends, "If you loved me you would 'do it' with me")
To this slave he actually sounds like a wheedling little teenage boy, trying to coerce something out of someone.
Truly though, no Dom should ever try and manipulate their sub in this manner. If she is against ATM, then she is against it. However, that being said, a Dom should also be able to gently lead their sub down a path of self discovery. If that is not a hard limit of hers, perhaps he should have "softened her up" by having cleansing rituals before play and then introducing her to the idea gradually. Say perhaps an inserted finger that he then touches to her lips?
This slave certainly has no problem with ATM so long as she is clean, but it is understandable to her why others might have issues with it.