If you post a gif of two guys having anal sex with one another, I think you can safely assume they're not straight.
I'm not sure why that needs to be explained to you, but here we are.
It appeared to me as though they were mimicking sex. I found it on a page with a theme similar to "straight guys doing not straight things", so again, I didn't really overthink it. I'm sorry if that gif ruined anyone's day, I truly am.