Hi Spaced
My daughter is 22 months old. My husband played football every Monday night, being away from the house for 2-3 hours including travel to/from. He asked me if I minded him continuing playing football after the baby was born.
I said I wanted him to carry on playing football, definitely. I know that he loves it, he gets time to spend with his mates, he keeps fit, and it's good to have some time to himself. He felt guilty at first and obviously missed his daughter, but I reassured him that it was okay and he should just relax.
On the flip side, my daughter understands her daddy plays football on Mondays and that it's girly night for us, so we always do a different routine and have lots of fun and cuddles together. She went through a stage of getting upset when he left (and he felt really bad) but she got over that, and now she accepts he goes to football, and even helps him put his shinpads on and helps pack his bag!
Some couples I know of, have the misfortune of having mum stuck home holding the baby whilst dad goes out on frequent, random drunken nights out. I'm sure your wife knows that you're wanting to pursue a meaningful hobby and not just swan off and not give a damn. Maybe your baby will take an interest in kungfu when they're older and will want to have classes themselves!