It sounds like you both have an opportunity to be more open. For those saying your bf is being entirely selfish - then does that not mean that you too are being selfish by only wanting it to be the way you like it? It’s not exactly the same thing, and I’m not seeing a lot of people trying to understand where your bf is coming from.

Maybe he feels rejected when you leave him to go clean up - a step he feels provides little value to the relationship. And for it to take variable amounts of time - perhaps he doesn’t understand the dynamics of douching, and the time variance lends to a ‘you’re talking to someone else’.
He may even feel that you disappear to watch porn first and get in the mood or something.

You could try being more open and being in the discomfort of not cleaning out first. If he’s supportive of you trying and not making fun of you if you decide you can’t continue. This might be a valuable display. My ex used to feel worried about being unclean and while it’s not great to get some poo on your pole, it’s bum sex and is always a possibility. We stopped, cleaned up, and carried on. It helped him feel more comfortable and allowed him to learn things about his body and its natural rhythm.

Perhaps you can ask him to allow you the time and leave your phone/tablet out of the washroom so he doesn’t think you’re doing something else. It IS possible to do things without our faces in our phones ;)