Sometimes during sex you do have to apologize. You're dealing with sensitive areas both physical and mental, you're going to fuck up, when you've fucked up it's nice to say sorry. That's okay. But people apologize too damn much during sex. (I know I do. Sorry about that.) They apologize for things they shouldn't.
Saying "sorry" can make someone less upset with you, but it can't make them happier. "Sorry I bonked you on the nose there" soothes the indignity of the nose-pain a bit, but "sorry you didn't have more fun" doesn't create any fun at all. And when you apologize for things that didn't bother the other person in the first place, you draw attention to those things and you sound insecure as hell.
So here are a couple things not to apologize for.
1) Your body.
They're fucking you, aren't they? Unless they're putting on a blindfold and handling your bits with tongs, it's pretty safe to say they find you attractive. Going "sorry, I have a big belly" can only go two ways:
-They actually kinda liked your tummy, but now are sad that you don't feel the same way.
-They didn't love your tummy but didn't mind it since the rest of you is so damn cute, and now you've drawn their attention to it.
Not only do the outcomes suck, but so does the implication: that your body is something you're doing to them. "Look out, I'm going to be fat at you!" Horseshit. Only the sleaziest teenage boys think that being attractive is a duty to others and being ugly is an offense or dereliction. If your body's somehow a problem, at least realize it's only your problem.
So he didn't get hard, you had weird pain and had to quit, she didn't get off, you came in ten seconds. Sucks for both of you.
So here's your options:
-Fix it. Take a rest and go a little easier this time, let your fingers finish what your cock started, ask her how she likes it and do just exactly that.
-Forget it. Welp, no sense going on if it's not fun anymore. Wanna watch a movie and cuddle? We can try again later if we feel like it, or not if we don't.
-"Sorry sorry ohmigosh that was terrible sorry."
3) Your limits.
Big one for me. "I'm really sorry, but I just can't get comfortable with crowbar blows, it must be such a letdown for you, sorry." Even if it's not crowbar-play, even if it's spanking or giving blowjobs or having your feet touched, you shouldn't apologize for a decision you're not planning to reverse. Maybe it really does disappoint them, but apologizing won't help and it makes the limit seem less firm.
The worst limit to apologize for is who you'll fuck/play with. Giving a polite no is nice; giving an apologetic no is annoying and misleading. Are you actually, literally sorry you won't fuck them? If so, it won't help them to know that. If not, don't lie.
I'm sorry for things I did and I know were wrong and you'll feel better if I say it. If it's not my fault, if I still stand by my actions, or if "sorry" will only be salt on the wound, then I'll be nice, but I won't be sorry.