Lust is not enough?
And what if you want a "better" reason?
I suppose it depends on whether or not the question, "Why do you want to have sex with me?" is important to you.
I have only been asked this question once, and it was the morning after. I had been incredibly intoxicated (inability to do simple math, extremely poor motor skills, terrible sense of humor) when we had sex the first time. The question was important to the asker, because he needed to be sure I'd have chosen to lay with him had I been sober. He asked at least three different ways, but it kept boiling down to the same question. "Why did you want to have sex with me last night?"
The simple answer, "I was really horny, and very attracted to you, and already naked in your bed." wasn't good enough for him. I suppose if he'd asked before we had sex, that might have been the end of it. The more complete answer was, "I knew it would eventually happen. Physically, you're my idea of male perfection. In our extremely brief acquaintanceship, you've already taught me new things. Your passionate, hedonistic approach to food, music, alcohol, sex, and just life in general is refreshing and novel to me. This isn't my M.O., but I've wanted you since we first spoke. In fact, as a measure to prevent myself from going this far on our first date, I neglected to shave anything, and wore ugly underwear, but as you can see, my measures failed. At the time, when you initiated, it just felt right." He liked that answer much better, and his response to it convinced me to only have dates with guys I definitely already knew I wanted to fuck, and to feel free to fuck them at the absolute earliest possibility.
Having had that experience, I can understand how it might be an important question to someone. "How or why did you choose me? Was it a clumsy, drunken accident? Or do you really want to have sex with me?"
Another thing occurs to me. A major motive of the asker might be to find out of the other person sees them as special, or just another in a long line. The person asking may want to be seen as special, or may be hoping that they are just another space on someone's to-do list. I was once cut off from physical contact with a man who discovered that he;d become a lot more important to me than I was to him. Even though I really liked him, I understood that all that could be between us was sex. We were sexually compatible, the chemistry ws ridiculous. But we had completely diametric ideas about long-term relationships. All I wanted was the sex, despite having intense feelings for him. But it would be lie to say that those feelings were not part of my wanting to have sex with him. He found out, and he backed way off.
Just as it may not be important information to one, I can easily see how it might matter to another.
