I agree with explaining about a time and a place for bathroom humor. I remember my kids asking why Daddy was allowed to use bad words and people on TV. I said 'they think that makes them sound like they really mean what they are saying and are trying to sound tough', but it's not polite and they shouldn't say bad words, especially not at school.

I told them it was OK for them to go into the bathroom and say bathroom words. They loved that!

I personally would totally ignore it. Yeah maybe use the broken record theory ....we use that talk in the potty...do you need to go?" Then ignore it.
I think this is testing grounds. Let's see where your limits are & how far they can be pushed. I have 3 kids & all have tested my limits.
Right now my 5yo is obcessed w: putting his middle finger up. Then giggles & looks to me. He's waiting for my reaction. I always ask him...did u hurt ur finger?" He knows from media, family, god knows where that putting it up means something bad. So he wants to explore that. Nothing like negative attention. So it's just a finger. No big deal.

I'd say relax & snicker about it. Take a deep breath when it gets under your skin. I know just that may be difficult.

My oldest got off the bus in kindergarten w/a brand new 4 letter word. Yeah the worst one. Lovely. That was hard to ignore too. We just didn't give the word any power & she lost interest.
Thank god!