Do i have the right to clean my flat mates bathroom if its the main bathroom to the house?

My room mate NEVER cleans - probably once every couple of months... me and my partner have an ensuit in our room so the main bathroom/toilet is for our flat mate... but he NEVER cleans it, i mean its that gross that when we get visitors they know to go to our ensuit if they need to use the toilet.

From time to time i will clean our flat mates toilet - for eg when i had my baby shower i cleaned it so everyone wouldnt need to go through our bedroom.

My flat mate reckons its an invasion of his privacy that i clean it, but isnt it more an invasion of our privacy that everyone has to go through our room to use OUR toilet? i dont mind cleaning his bathroom what so ever, i'm a stay at home mum so i get bored sometimes lol but seriously, if its a main bathroom/toilet to the whole house, shouldnt every visitor have the right to use it, and do i have the right to clean it?

I have lived with this flat mate on and off for six years, we've lived in four different houses together and he's been friends with my fiance since they were 3 years old - so he's no stranger