I think I'd be a little curious about that too. Perhaps she just thinks it's faster that way than giving him a bath. I'm sure that's her thinking, but i do think it's time for a young boy to begin taking his own baths/showers by himself.
Are you two arguing about this? If so, it's not healthy for you to argue in front of the child. It will get him traumatized about sex and his own body so stop the arguing!
Why don't YOU take over the bathing of your son? Have him take his showers with YOU? It's more natural for the guys to shower together anyway and would solve the problem, right?
Hopefully that will solve the problem with the bathing. God bless and take care. Hugs, Lee