I've ALWAYS wanted to do that to a girl I'd be potentially dating seriously.
I think I got the idea from that fucked up movie "Kids" where the guy wanted to fuck a virgin in the ass for her first time with a guy.
I think it would be so cool to really dig a girl, it be mutual, fuck her in the ass before vaginally, get married.... and always be able to say that you gave her butt love before she took you in the pussy.