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 insult is the hypocrisy, not the gayness.