I don't get it.
I think it's addressing the rumor that black men have huge cocks, making it difficult to walk after having sex with them.
Madeline Kahn in Blazing Saddles. "It's troo It's troo."
Well obviously I didn't get it either. I thought it was some racist stereotype about black men beating up their girlfriends and I was going to have a go at the poster. Now I'll just crawl back into my ignorance box.