She was holding back her virginity for that special someone she became romantically involved with. This was a hallmark trait of her character -- it defined her. You don't find it more weird that at some point she is just gonna say "Screw it!" and give it to Max without any romantic feelings?Well, it was a bit too weird to write romance story after anal with Lisa.
Also, is there something about anal that prevents future romance? There are plenty of people who have been friends with benefits/fooled around and became romantically involved after the fact.