Both can be funny in the right context. Furthermore, using "straight" as an insult doesn't uphold any kind of systemic oppression, so I have a hard time reading it as anything other than a joke.
The most problematic thing about it is that it is reminiscent of homophobia, but y'all gotta learn to make the difference. Giving bigoted straight people a taste of their own medicine in a context where it cannot possibly hurt them (unlike actual homophobia) is funny.
Insults are a conversational, social, and emotional weapon. Weapons have valid uses, for example in self defence.
The use of many weapons has effects beyond the immediate violence they do. For example if you buy a gun from a cartel, you help them scale their business and sell more guns, leading to more gun violence. Likewise if you use slurs against marginalised communities, you reinforce the stigma and make the slur more potent, increasing the violence done later. You're emboldening an infrastructure of violence in both cases.
If you insult someone by calling them gay, you hurt them and you also hurt all gay people. But if you insult someone by calling them straight, you hurt them without hurting all straight people. Because you are not emboldening an extant infrastructure of violence.
Listen to your mental gymnastics. Just don't insult anyone with their sexuality. Period. You could take it even farther and just say "don't insult anyone" Seems pretty safe to me.
Remember how people called homosexual people, "gay", as an insult and instead of making it a bad word, like the n word, homosexual men were like, "You know what? Fuck you. I am gay. And I'm not just going to let you call me gay. I'm going to call myself gay."? And this word that could have grown to be as powerful of an insult as the n word became empowering to the people it was intended to shun. I always thought that was a baller move.
I have always thought that people that want to insult me are going to do it, no matter what, and people that want to be respectful to me, are going to do it no matter what.
My trick is I don't let what people call me bother me. I was raised with the old saying "Sticks and stones may break my bones, but names will never hurt me".