I notice that a lot of people who claim to be sex positive seem to have a problem with rejecting people or asserting boundaries in a truly sex positive way, especially in situations where they feel disgust.
These situations are the ones where it is most important for us to embrace sex positivity, because they're the situations where we are most likely to send (perhaps unintentionally) a sex-negative message.
Examples of Sex-Negative Rejection:
Kinkshaming - "Eww, gross, you are into that? That's disgusting / wrong."
Person shaming - "You are attracted to me? That's gross / you are wrong for being attracted to me."
Identity shaming - "Eww, a (person of this gender / race / ethnicity / (slur for neurodivergent person) / trans person / (insert other identity here) is into me, that's disgusting / wrong."
Equating feelings with morality - "I feel uncomfortable with what this person said or did, therefore what the person did was wrong."
Feelings of discomfort are often a tip-off that someone has done something boundary-crossing, but they can also originate from cultural sex negativity and bigotry, which often gets directed at LGBTQ people, kink, neurodivergent people, and other marginalized groups.
If you want to reject people or assert boundaries in sex positive ways, you need to examine your feelings and find a way to express yourself without the sex negativity.
Sex-Positive Rejection or Boundaries: (with added optional explanation)
"I'm not into that. (If you're into it that's fine, but I'm not willing to do it with you or with anyone.)" - A simple, fine way to say no.
"I am repulsed by the thought of that / feel uncomfortable talking about it or hearing you share such thoughts. (If you're into it, I'd like you to keep those thoughts to yourself. It's fine that you brought it up this time, but don't bring it up again.)" - A way to express your feelings of discomfort without shaming the person.
"I'm sorry I'm gay / straight / asexual / I'm just not into you in that way. (but thanks for the interest or offer / best of luck finding someone you can connect with)" - You don't ever need to share anything about your sexuality when rejecting someone, but sometimes it can make it less likely the person will interpret your rejection as shaming them. Thanking the person for their interest and/or wishing them luck in seeking future connections is another way of achieving a similar effect, especially when you don't share about your sexuality (or if you are bi or pan and thus don't have this "out".)
If people are rude to you? You don't need to be nice to them. But still, you cannot be sex positive if you shame them for their kink or identity or merely their expression of interest, rather than specific examples of rudeness or boundary-crossing behavior.
Sex-Positive Ways to Call Out Rude or Inappropriate Advances:
"It's totally inappropriate for you to express your interest in me because you're my supervisor / teacher / I'm under the legal age of consent and you know it / I've already told you I'm not interested and not to ask me again / you know I'm in a monogamous relationship / you're in a monogamous relationship."
"I barely know you and it is totally inappropriate for you to make a sexually explicit comment or ask such an explicit comment of a stranger."
"I've already told you (repeatedly) that I'm not comfortable with you saying / doing / asking that, and it is completely inappropriate and rude for you to continue."
Note carefully that none of these examples shames the person for having a specific kink or for being interested in you, nor do they reference the person's identity. They are specific in identifying concrete reasons for your negative reaction so they remove all possible doubt or speculation that you might be kinkshaming or identity shaming, and they make crystal clear exactly what the person did wrong.
Note also that they do not make an appeal to emotion or imply that the person is wrong merely because you feel uncomfortable.
P.S. These are also the situations where it is totally warranted to seek external help such as friends or authority figures, to make sure you stay safe, ensure someone leaves you alone, and/or to post publicly about what happened in order to warn people about someone overstepping boundaries, harassing people, and/or seeking inappropriate connections.
It is not appropriate to post negative things about a person or speak negatively about them behind their back, merely because they were interested in you and you rejected them, and/or they expressed interest in a certain kink that you found repulsive. Again, that is sex negativity.
I hope this can help others both to learn how to assert boundaries in a sex positive way, and protect people who get subjected to sex-negative shaming, so that if people do this to you, you don't let it get to you.