why is he mad?

Today my boyfriend and I had a little fight but it was really nothing we were both basically annoyed at each other but then when he came over to me to make up, he said let's go fuck in the shower to which then I said I don't really feel like it right now. I have had a long busy day and am tired so I just dont want to. But he got really really mad. ( I also told him that it was a stupid reason to be mad at me) but like why?!? Omg!!!