The idea that sexual attraction and sexual desire are completely different types of emotions is something that asexuals have hashed and rehashed over and over again. Seemingly, one of the big difficulties in coming out/having any discussion about asexuality with nonasexuals, is bumping into the association that sexual attraction has with sexual desire.
Certainly they can intersect. If you are nonasexual or in a gray area (like I am), you may want (desire) to have sex with someone because you are attracted to them. But maybe there have been situations where you have not been attracted to someone and still wanted to have sex with them anyway. Maybe you were experimenting, maybe you enjoyed being desired, maybe you needed comfort, maybe you were having a night on the town and wanted to have some fun with a stranger, maybe you thought that sex would be physically plesaurable, but you weren't attracted to the person you had sex with.
Here's a scenario. A woman has multiple sexual relationships with men, and then one day she realizes that she is a homosexual. Homosexual lesbians experience sexual attraction to women, and they lack sexual attraction towards men (I'm talking cissexual men here especially). So what are some possible reasons that the woman in this scenario could have wanted to be sexual with men? Experimentation, physical release, assumptions about attraction, to be more connected to her partner, because she thought it would be the thing to do, opportunity, etc.
It's important to understand what motivates someone to have sex, rather than assuming that all sex is based on sexual attraction. The above scenario can also illustrate another common issue faced by LGBTQA persons, which is the "I cured you!" dilemma. It doesn't matter if you are asexual or not, if you don't understand the difference between attraction and desire, then anyone can claim that your sexual orientation is false and you will not be able to defend yourself.
If the woman in the above scenario were a so-called gold star lesbian and had never had sex with a man, she might be able to say that she knows she's a lesbian because she's never had sex with a man. One problem with this argument is that it equates a lack of sexual experience with a lack of sexual attraction. Therefore, a counter argument could by someone who is ignorant of the difference between attraction and desire. "You just haven't had me yet," or "if you tried it you'd change your mind," are both instances of assumption that experiencing sex and being sexually attracted to someone are the same thing. It's also typical to argue that if a heterosexual man is experimenting sexually with another man then he must experience homosexual attraction that he is hiding from himself/others. What if the opportunity arose and he was just horny, or he was curious, and doesn't feel sexually attracted to men at all? If he's a queer ally, is it really fair to say that he's in denial if he still identifies as heterosexual? Are we so insecure with our own sexualities that we have to accuse him of being gay or bisexual, hypocritically using the same arguments made by homophobic people?
Thus, even experiencing sex (nevermind wanting to have sex for any reason) gets confused with sexual attraction because people have not learned the importance of differentiation. If you think that experiencing sex and sexual attraction are the same thing, then you must also think that a victim of rape was sexually attracted to their rapist. Totally ludicrous. Even if a rape victim is in a relationship with their rapist and is sexually attracted to them, that doesn't mean that they will always desire sex because of this attraction/that rape is excusable. This conflation also lends credence to the argument that rapists always commit rape because they are "overwhelmingly sexually attracted" to someone. Again, totally inaccurate. Rapists desire sex because they want to control/express violence, and whether or not they are sexually attracted to the victim is irrelevant.
I bring this subject up because I am an asexual-identified person who sometimes desires sex, even though I either do not experience sexual attraction or experience low-intensity sexual attraction. In any case, I think it should be clear that sexual attraction and sexual desire are incredibly important concepts to differentiate in coming to understand sexuality. Sexual behavior is not an indicator of sexual attraction, even if one often follows the other.