I'm gonna answer this seriously since you put seriously.
Honestly, I think about a lot of things during sex. Besides it feeling good and feeling out when I should cum, a lot of its been negative sadly.
I would usually just think about how disappointed with my girlfriend I was sexually. I would think about previous relationships and how they did things that were really exciting and felt amazing and how my current girlfriend does the bare minimum to have sex. She's getting better about it but she's still unpredictable as far as sex goes. Sometimes it's enjoyable, sometimes it's absolutely frustrating, rarely is it memorable and special.
Other than the general distaste for my current girlfriend's sexual appetite and experience I would be wrapped up in "is this working for her? She's not giving any good signals." There's far more serious things I think of too. Though it's harder to recall them. I would think about intense thoughts sometimes that would pop in just for a moment. Like "what if she got pregnant? Is this who I want to keep having sex with?"
Before that? Sex with other partners was always a nice tightrope walk of my enjoyment and her enjoyment. It was a battle but the end was always both of us ending in victory and enjoyment. I would think about how they looked and how they were having sex with me, and if I'd remember it later. I definitely have a highlight reel in my head of great sex.
I know if my girlfriend saw this she'd be upset, and I have talked to her about it many times. But I chalk it up to her being inexperienced with long term relationships and not used to actual intimacy. But we're coming up on 3 years and it feels like wayyy too long for some things. For example she used to not want to make out which really bothered me since I felt self conscious about it. Turns out she just didn't know how to do it right. She just assumed she didn't like it. But instead of learning she just stopped. Which writing out loud is incredibly stupid and shitty to do.
After I showed her the ropes she loves it but still. 3 years? That's fucking embarrassing for a partner to put someone through.
But back on track. Those are thoughts I have during sex.