The first thing that comes to mind is... 4th finger on the left hand? Finger Blasting the V with that? He might not recognize it, but that's a good guaranty for birth control. Plus, dude must have some weird flexibility.
If she asks for a third finger it can be hard to avoid putting no. 4 in there. It's either 2,3,4 or 1,3,4. 1,2,3 is awkward, and anything involving 5 but not 4 is asking for a broken pinky.