It's as if they gradually sensed their mutual tendencies for perversion, coupled with latent mutual attraction towards each other ... And It Is Glorious.
A connection born via mutual masturbation is a truly deep connection that leads couples together.
Also maybe they should've just masturbated together if they share such a connection, I'm sure that's another way they could've gotten together.
That or he just worked up the guts to confess. But hey, things turned out great and I'm happy for them.
SCIENCE!