Simply put, there's more to wanting sex than just "yes" or "no". Sometimes that yes is a yes but not for yourself, sometimes that no is so intensely no for good reason. So, let's do this, get right in. There's a set of four terms, sex-favorable, sex-indifferent, sex-averse, and sex-repulsed, that I felt deserve their own blog post. Because these can be you even if you're not ace. Now, if you're wondering, are there romance-favorable/indifferent/averse/repulsed as well? Yes. More or less replace the concept of sex with romance and yes, the same.
So, sex-favorable means that within the scope of either relationships or other comfort zone, you're up for sex. Maybe you even initiate it! You might enjoy content that is sexual in nature, and you might engage in sex because it's fun, or feels good to you, or whatever. You might be favorable to only a few sex acts, while being repulsed or indifferent to others. So why not say “sex positive”? Sex positive is the stance that consensual sex is a net positive, it's a social/political stance instead of an individual stance.
Sex-indifferent is exactly what it says on the tin; you can take it or leave it. It's probably not something you think about, and it's not something you're likely to initiate. You aren't saying “hell yeah!” nor “hell nah!”, more like “meh, sure, I guess, maybe?”. Generally speaking, if a sex-indifferent person is having sex, it's for some outside reason, not an internal interest or drive for it.
Sex-averse is not wanting sex, but not because it disgusts you, rather that it makes you uncomfortable. It could be because of sensory hypersensitivity (i.e. feeling too much when someone touches you), it could be because the idea of being that close to someone causes discomfort, or that the general activity of genital to genital (or genital to whatever else) contact makes you entirely anxious (so that pleasure specifically can't happen in any way, not just “a case of nerves”). Or really anything that makes it so that, physically or psychologically, the act of sex or even the idea of the act of sex gives you such discomfort that you are fully uninterested in engaging in it.
Sex-repulsed is where the idea of sex, usually you but possibly the idea of sex itself involving anyone literally disgusts you. It makes you gag, you feel physically ill/nauseous at the concept of contact in that manner. Maybe it's the fluids, maybe it's the closeness, maybe it's having someone else's parts close to you, maybe it's all of it, or some other reason. Regardless of the reason, even the suggestion of sex makes a sex-repulsed person queasy. Now it could just be certain sex actions, or certain people (a hetero guy being repulsed at the idea of sex with another guy, a lesbian feeling repulsion at the idea of sex with a man, etc), but the result is the same; sex in that case is not just a no, but an active detriment.
So why does this deserve it's own blog, separate from ace and aro explanations? Because while these are used by the ace community in specific, they can also apply to any orientation, and their general usage can lead to them being more widely accepted. Being ace is still one of the most medicallized orientations because it's so widely misunderstood, and because it's simply assumed that everyone feels the same amount of sexual attraction and the same amount of willingness and interest in sex, and if you don't there must be something wrong with you. But there are heterosexuals who are sex indifferent, and they're also diagnosed as having some “problem”. There can be people who are both pansexual and sex averse, and that could be seen as an issue as well. In fact, anything other than full agreement in heterosexual interaction can potentially be misdiagnosed as an issue, and aces with not feeling any sexual attraction can get a double dose of trouble if they admit to being repulsed or averse to sex as well. So awareness helps.
A short note, as stated at the start: just replace the actions of romance with the action of sex and you've got the romantic understanding of these in brief for that kind of thing; the short and sweet of it is that romance favorable would be totally up for romantic things regardless of if you feel romantic attraction or not, indifferent would be just not caring or initiating in any way, averse would be “I would really rather we not do the romance, it makes me uncomfortable”, and repulsed would be actually feeling physically ill and grossed out at the idea of romance.
If this is helpful, pass it along! If you have a question feel free to ask in the comments (at least until this Friday, August 12th, when my membership to this site expires). If you have a question after that, feel free to ask at the Twitter account listed, just know I only check that account every 2-4 weeks so give time for a reply. 👍