I guess even she has her limits. Maybe that's one of them. It's true that she's very very slutty, but somewhere she has to set the bar of unacceptable.
In fact she doesn't do creampies with anyone but her husband, and no anal either. Those are clearly other limits. Once upon a time she wouldn't even kissed other guys,... although that limit is long gone.
Mind you, she has no problem licking guys' ass holes thoroughly. It's one of her trademarks in fact. If she does that, I'd say she could do anal creampies no problem. A matter of time I guess.