DISCLAIMER:This joke is homophobic. "Trump being embarrassed for people thinking he had a gay relationship makes him homophobic. Affirming that embarrassment by making it the butt of the joke makes the joke homophobic"
this article explains it well: https://newhumanist.org.uk/articles/5082/the-dangerous-stereotype-of-the-self-hating-homophobe
The punchline is that they are sleeping together despite every other indication being that they hate each other. It would be equally funny if he was rumored to sleep with hillary clinton, and in fact those jokes were really common in the 2016 election. here are some examples
Oh I get it. You don't think humor exists at all. Got it.