I'm tired of people saying that having a headache is not an excuse not to have sex. Look, I get that an orgasm can relieve a headache, but all that means is that if I want to have sex, a headache shouldn't stop me. If I don't want to have sex, for any reason, I shouldn't have to have sex. The idea that there are bad or invalid excuses only reinforces the idea that you need a "good reason" not to have sex, and if you don't, you'd better spread 'em for your guy.
Now, there's traditionally been a missing discourse of desire in our society - basically, it's been assumed that women do not want sex, and merely have sex out of obligation or wanting to please her partner, or because she's pressured. We know that some women still have sex mainly for those reasons. In order to combat this, many sex-positivists instead assert that women actually really want sex. A lot. All the time. Now there's a myth that women are really horny little monsters who are ready and willing all the time, unless they adopt a pure lifestyle, in which case they really need to be liberated because they must be oppressed. Because a healthy, sane woman would never choose abstinence, right? This new myth assumes that if a woman doesn't want to have sex, there must either be something wrong with her, with the guy she's with, or with the relationship - and if that's fixed, she'll fuck away!
This only opens the door for coercion. If it is assumed that all women want sex all the time, it only gives merit to the idea that a woman saying "no" is only trying to play hard to get, or trying to get the guy to work a little harder, or just following a social script - she doesn't really mean it. And if she does persist, a little work on his part to get rid of whatever problem she's having is really all it takes. Once the problem's resolved, her excuse is gone, right? The new myth that women are extremely sexual beings also poses coercion as a favor to the one being convinced. Often someone will legitimately believe they know what's best for the other person, they "know" what will feel good for them even if the person insists they have no desire to do it. There are men, my ex included, who feel that a woman rejecting sex doesn't fully understand what she's turning down, and that she'll thank him for talking her into it later. Women are actually encouraged to talk their male partners into trying anal play, even if they vehemently reject the idea.
I'm gonna get a little personal here. I get that anal play does feel good for a lot of men, and it's a shame so few are closed to the idea because they feel such sex acts are gay and emasculating. However, if my partner says he doesn't want anything in his butt, I'm not going to try to coax him into trying it, nor am I going to stick my finger in there when we're fooling around, sure that once it's in there he'll change his mind. Why? Because I know firsthand how it feels to have someone penetrate you without asking first. It's not fun, the words "disrespected," "violated," and "raped" are among the first come to mind. Also, I can understand that just because a sex act feels good for a lot of people doesn't mean it's right for everyone - for some, even a mainstream sex act can be more painful or just more weird than pleasurable. To each their own, leave it at that and move on. If they change their minds on their own, they can say so themselves.
Coercion is never okay. Even if you mean well, even if you really think your partner will thoroughly enjoy whatever you're trying to get them to do, it's not okay to coerce them. No means no, it doesn't - it at least shouldn't - have any other inherent or assumed meanings. No doesn't mean "try harder," it doesn't mean "I really shouldn't," it doesn't necessarily mean "I don't like you" or "I don't want to date you" or "I want to break up," it just means "I don't want to have sex." And it's entirely possible for a sane, emotionally and sexually healthy person to simply not be up for sex once in a while. Once we recognize that everyone's sexuality is different - that some people want it more often than others - we will understand that the word "no" needs to be taken much more seriously than it is, and that one need not give a "good reason" why not.