Even though I'm late, I will put in my experience. My SO actually enjoys at least a digit or two almost every time, at least up and to the point of begining her O. Then out and let her bask. However, it must also be shared that during full on "butt sex" (which is maybe once every 6 months of so) she actually has had some of her most intense O's... Her first 2 times, full on squirting, actually came at the hands of the "dirty deed"...
As for me, I agree with many in that the sensation itself is not as nice as the natural entryway. Butt the taboo excitement and the sheer fact that she only asks for it when she is OOC (out of control) tends to creat memories for a lifetime! I have never been the instigater and I think that has a lot to do with it. It is a timing, heat of the moment type thing and if her body is asking for it, I do my best to please!