I was having a bath last night and he came in and used the loo (no.2). I shouted and asked what he was playing at and he said he has always done this with partners and vice versa as have plenty of people he knows. He stayed for a couple of minutes until he had finished and left laughing.
For context - together 4 months, he was staying at mine last night.
He has said he won’t do it again but I feel a bit humiliated by it tbh.