I can definitely believe that bringing up one's virginity prior to any sexy time would result in some guys shutting it down. However, that guy was face deep in lady bits and then just drops an "I'm not into virgins" and walks away. No effin' way. I don't know if I could be convinced that there are more than 50 men--especially 23-25 year-old men--on the planet that would walk away after getting that far, especially considering how attractive Shoshanna is. The entire scene was hilarious (Shoshanna starting to laugh as he was going down on her and then stopping herself), but that was ludicrous.
TMI Time! I was 20 and crawled out of a bed naked rather than be someone's first for the exact same reason, so it's not only plausible it's as realistic as it gets.