Cramping is your body's way of betraying you whenever it wants, because the fleshy husks that house our brains are evil. Everyone is worried that their gymnastics-gold-medal-level sex is going to result in a charley horse. It's a risk we have to take.
2. What their butthole looks like from that angle
It looks like a butthole and everyone has them, so calm down. Unless you have a third eye down there, you've got nothing to worry about.
3. If they're sweating too much
It's sex. If you're not sweating, you're just lying on top of each other with a penis in a vagina. Plus, sex sweat is different than normal sweat, so it's not like you're all gross-just-came-from-the-gym sticky. You're glistening with dew.
4. If they're taking too long to come
No. No, you're not.
5. If their partner noticed they just totally zoned out for a little
Unless you stopped moving like you just got tagged in freeze tag, no. They didn't. You were probably pumping away while your mind wandered.
6. If their partner thinks their body hair (or lack thereof) is weird
Even if, for argument's sake, you had some weird body hair, they're already having sex with you, so you passed the weird body hair test.
7. If they're breathing too loud
Your partner probably can't hear you over the sound of their own loud breathing.
8. The never ending threat of an accidental fart
A great trick is to just scream really loud when you release it. That or just acknowledge the fact that these things happen, have a laugh about it, and then move on.
9. Whether their partner is secretly bored
You can't spend your whole life worried that people are pretending to like you. No, they're not really, really good actors having fake-good sex with you. They're boring you because they like you and think you're hot.