What was he doing

My guy came home from work and went to shower like always he works in a smelly shop. While he was in shower i walked in to ask what he wanted to eat ( it’s never a problem for us to come in bathroom with other person) his penis was hard. I asked him was he touching it he said no I don’t care if he was he know So I said then y are you so hard he says oh that’s normal 🙄🙄 no it’s not I been with him for 4 years and never seen it like that from a shower. Also the bathroom door was locked it’s never locked. Could he have been pleasuring himself? If not then y lock the door