blimey, some fun sponges on here...
OP, if you're interested in the flirt, silly/clumsy as it is and want to encourage, you can respond with a jokey comment like
"You'll have to shake your tail feather for that privilege"
If you're not bothered and want to slow it down without completely shutting it down you can say "One day if you're a lucky duck"
If you want to shut it down completely then something like "Haha, cute idea, but no chance!"
It's a flirt, respond in kind or don't, depends if you're interested, and if you're not, then stop talking to him.
As for others, i tell my male friends i'm going for a bath/shower all the time in conversation if that's why we're ending it, same with going to make lunch or running errands.. are grown adults really treating the phrase 'going for a bath' like a 'i'm getting naked' invitation?