Through the years, I found that analingus has always led to anal sex. It has never failed me. I find that if a girl will let you stick your tongue in her ass, she will let you fuck it as well.
I find that if a girl won't do anal, she won't let a guy tongue-fuck her ass, either. Once you help her get over that taboo, she'll open the anal floodgates. Once she first enjoys it, the rest will be a matter of time. She will do ass to mouth and certainly become marriage material.
