It is one of those things that is either extremely erotic or gross depending on who your partner is and the events leading up to intimacy. I will say that if I am very physically turned on by a girl, and she wants me to do this for her and genuinely gets off from it (both physically and psychology), it really is hard to top that in terms of sheer intensity of a turn-on. It is a very animal type of thing (think about it...Animals are always interested in putting their faces in that area of one another) so it is not so much of a lovey/sweet thing as it is acting on an impulse that seems weird against a civilised backdrop. If I am extremely physically attracted to a girl, I get a dark curiosity about this kinda thing in the same way some guys might want their faces between breasts in a fantasy.
I only want to do it if the girl genuinely gets off on it, though, not if she is going along because I like it.
As well, it plays on our insecurities, and a lot of people get sexual gratification teetering at the brink of embarrassment. That is pretty much accepted as the dirtiest part of the human body - the be all end all of perverted exploration - and your partner putting a tongue there can be seen as pretty much the ultimate sign of being comfortable with your body and every inch of it. I don't know, I have been in relationships where we did that and others where we did not, and it is hard to exactly put my finger on it, but being out to dinner with several other couples, there is just this unspoken sense of close(r)ness between you two when you like to put your tongues on each others assholes.
That last paragraph hit on the psychology of it, but it is also a highly-pleasurable activity for the receiver. And call me sick in the head, but if I am attracted to the girl enough, I swear she will taste good down there.
