You can decide you don’t want to have sex at any point for absolutely any reason. In fact, if you feel even a little uncomfortable, communicate that to your partner. It doesn’t matter whether you’ve given consent or not, you can always change your mind and choose to stop (more on that here).
Some things you can say:
“Can we pause? I’m feeling a little off / uncomfortable”
“Hey, I’m tired. I don’t think I want to keep doing this.”
“I know I said I wanted to do this, but it doesn’t feel right. Can we hold off for a bit?”
“It’s been awhile since I’ve done this… so can we take it a little slower? Maybe tonight we just stick to making out.”
“Can we rearrange? Can we try it more like this?”
“Hey, I think we’re both a little drunk right now, but here’s my number. Let’s meet up again tomorrow night.”
If you’re with a long-term partner, consider coming up with a safe word or signal for when you’re feeling a little uncomfortable or want to do something different. For example, say “yellow,” and that can be a cue for your partner to stop what they’re doing and slow down to give you some space to process. Or tap them twice on the shoulder to switch positions. You don’t have to justify it or explain. Just make sure you have a way to communicate with your partner when you’re not interested in doing something that might be difficult to say out-loud.
Important note: if your partner tries to pressure you into doing something you don’t want to or makes you feel bad about the boundaries you’ve set that is a huge red flag. Giving affirmative consent is a clear, voluntary decision to participate in a sexual activity and not a silence or lack of resistance (more here). Whether it is a casual hookup or a long term partnership, you deserve respect.