First off, sex positivity isn't (necessarily) about sexual behavior. It's not about how many people you shag, nor in which position you shag them, nor what you do with them after you're done shagging. It is possible to be asexual and still be sex-positive. And on the flip side of the same coin, it's possible to have a dozen lovers with whom you shag in the Monkey And Crane With Hand Grenades On A Trapeze position in three-day thirteen-way marathon orgies while still being sex-negative. The number of partners you have doesn't define sex-positivity.
It's also not about having an anything-goes attitude toward sex. I once had a person online claim that being sex-positive essentially means little more than embracing total hedonism as the only sexual ethic; his argument was that the term "sex positive" means seeing all sexual activity as a positive thing, and that since nobody actually does that for real (few people, for example, are willing to make the argument that nonconsensual sex is OK), sex positivity doesn't actually exist.
And sex positivity is not about dismantling cultural norms. Just as it is possible to be asexual and still be sex-positive, or to have wild kinky sex with twelve partners at the same time and still be sex negative, it is possible to be married in a monogamous relationship with one spouse and live in a suburban house with a white picket fence while still being sex positive. You can not tell whether someone is sex positive or sex negative simply by looking at their sexual arrangements.
Sex positivity at its core is simply the recognition that there is more than one "right" way to have sexual relationships. It is an acknowledgement that human sexuality is incredibly diverse, that different people have different tastes and relate to sexuality in different ways, and that as long as everyone is having sex with consenting adult partners, there is nothing intrinsically wrong with sex, regardless of the way people relate to it. In short, it's a deliberate refusal to place one's own sexuality on a pedestal and proclaim it the "right" way to have sex.
Sex positivity is not about how many people you have sex with. I've talked, for example, to a person who is polyamorous and who has several sexual partners, who believes that having sex with anyone you don't love"degrades" and "cheapens" sex, and is therefore wrong. Sex positivity embraces sex for a wide variety of reasons. Some people like to have sex simply for pleasure, without attaching expectations of emotional commitment to it. That is OK, provided that the sex is freely consented to by the people involved.
Sex positivity is not about having sexual relationships in any specific form. Some sex-positive activists see traditional pair-bonded monogamy as inherently disempowering and therefore bad, and say that being sex-positive means seeking to overturn heterosexual monogamy. Monogamous relationships are absolutely right for some people; there is nothing wrong with choosing to have sex with only one other person, just as there is nothing wrong with choosing to have sex with two, or five, or more.
Sex positivity is not (necessarily) about sex for pleasure. Nearly every person I've talked to and every Web site I've read about sex positive relationships focuses on sex as a natural and healthy thing to do for pleasure, which it is. But people choose to have sex for a number of reasons. Sex for procreation is a valid reason for sex. So is sex for profit. While it may make many folks squeamish, exchanging sex for money is a perfectly valid reason to have sex. Believing that a person who engages in prostitution is inherently less worthy than a person who has sex only for pleasure is not sex positive. Believing that all people who exchange sex for money are always disempowered and are always victims is likewise not a sex-positive attitude. Fundamentally, people have the right to control their bodies, and this extends to the right to choose when, how, and for what reason to have sex. Absent any coercion, sex for money, in any form from traditional prostitution to cam sex to production of porn films, is just as valid as any other consensual form of sexual expression.
Sex positivity is not (necessarily) about gender identity or orientation. I've heard the claim that all people are bisexual, and that anyone who is sex positive must embrace the notion that sexuality should ignore gender identity. The fact is that you can no more say "all people are bisexual" than you can say "all people have two legs." Some people are attracted only to folks who have the same gender identity they do, some people are attracted only to folks who have a different gender identity than they do, some people are attracted (to varying extents at varying times and in varying circumstances) to members of many gender identities; while there is not necessarily a 1:1 correlation between gender identity and biological sex, there often is; and all this is a normal part of ordinary human variability. It is not necessary to identify as bisexual or pansexual in order to be sex positive. It is not necessary to have sex with partners of different sexes in order to be sex positive. Heterosexual cisgendered people can be sex positive.
Sex positivity is not about enshrining sex as a sacred act. I've spoken to folks in the tantric sex community who claim that in order to be sex positive, one must embrace the "spirituality" and "sacredness" of sex. For some people, sex does create feelings of spirituality, and that's fine. But it doesn't have to. Sometimes, it's just about the orgasm. Sometimes, it's just about friendship. Sometimes, it's just about finishing the movie. The fact that it can be spiritual doesn't mean it has to be.
Sex positivity isn't (necessarily) about hedonism. Some people seem to have little or no drive toward sex; other people seem to take no particular pleasure from sex. A person may choose not to have sex, and that choice is just as valid as the choice to have sex. A person may choose to have sex for reasons other than physical pleasure--for reproduction, say, or for the gratification of a partner, or to scratch a biological itch--and that, too, is just as valid. A person is free to limit his or her own sexual expression, and provided that person does not then impose that limitation on other people, that person can still be sex positive.
At the end of the day, people have sex for a lot of different reasons, none of them intrinsically more "right" than any other. If you look at two (or three or fifteen) consenting adults having sex and say "Hey, what they're doing might not be what I would do, or might not be happening for the reasons I'd do it, but that's still a valid form of sexual expression nonetheless," that's probably a sex-positive attitude. The instant you start attaching thoughts of "They are more debased than I am because their reasons for having sex aren't as good as mine," you've stepped away from sex positivity, seems to me.
We're all human beings, which means we all come factory-equipped with a tendency to see our own motivations as purer and more evolved than those of others--especially when we don't understand why anyone else wouldn't do something the way we do it. To be sex positive, it is necessary to take a conscious step away from that natural human impulse.