Different preferences, whether personal or cultural, are important for the information they provide and are no more or less important than hair color or family history, unless people are trying to figure out what to eat together. Some people prefer to eat with the same person indefinitely, others prefer to eat in a group and still others eat with a variety of partners as the mood suits them and nobody is ever forced to eat anything or with anyone. Each person is an expert in their desires and needs around food and their choices are respected. Now go back through the last paragraph and substitute sex for food and have sex for eat. Sex-negativity keeps us us from moving towards this world.
Some people also talk about erotophobia, the fear of sex.
One common definition of sex-positivity is that it's the belief that sex is good. However, it can be more useful to reframe our definition of sex-positivity from sex is a positive thing to working towards a more positive relationship with sex. There are several different reasons for making this shift. First, it acknowledges that sex is neither good nor bad. Just like food, the benefit or harm comes from what you do and how it affects you.