Sex - no, you don’t have to have sex with anyone you don’t want to, or when you don’t want to.
Of course, as with most things, but that doesn’t mean that you can’t feel pressured to do something because you want your partner to be happy, and they want to do it. You always have the right to refuse, but “I don’t want to do X, but I know my partner does, so I do want to do X because I want to make them happy” is an intrinsic downside of a relationship. In turn, you should always be able to say no, and your partner should respect that because they know it wouldn’t make you happy, and that’s more important than getting what they want. “You can just say no” is true, but it also bypasses the entire point of the conversation around what makes being in a relationship difficult.
Of course, as with most things, but that doesn’t mean that you can’t feel pressured to do something because you want your partner to be happy, and they want to do it. You always have the right to refuse, but “I don’t want to do X, but I know my partner does, so I do want to do X because I want to make them happy” is an intrinsic downside of a relationship. In turn, you should always be able to say no, and your partner should respect that because they know it wouldn’t make you happy, and that’s more important than getting what they want. “You can just say no” is true, but it also bypasses the entire point of the conversation around what makes being in a relationship difficult.