Sexual interest is not related to the real possibility of having sex with someone....
I gather from several posts in this thread that, for some men, that's true enough. I am certain, though, that it is
not true of all men.
Why would you have sexual interest for a porn star (or a lesbian ) if it is very unlikely she will want to have sex with you?
If it is evident to me that someone is not at all interested in having sex with me, then that has an extremely strong tendency to very quickly and very effectively shut down any sexual desire I have for them. I can't think of many times that it's been otherwise -- and the few times when I recall it might have been otherwise all took place when I was younger.
Do keep in mind that many men do not think of lesbians as an impossibility. Lesbians are often seen with a possibilty to become bisexual at some point.
So I gather from several of the responses to this thread. Do you think such men are being realistic?
Would you still have sexual interest for an heterosexual girl even if she said she doesn't want to have sex with you? Surely.
Surely not. Not so long as she lacks interest. And if her lack of interest is for any reason especially blatant or decisive, then I'm likely to thoroughly dismiss her as a potential sex partner.
Near as I can relate to what you're saying is to recall how it was like when I was 16, 18, 21, or even as late as my mid twenties. Back then, I was often horny enough to have sex with anyone who let me -- regardless of how unenthusiastic they were. I think that level of horniness is pretty much natural early in life.
I imagine, if someone had asked me the OP question back then, I would have answered it much more like you have. So, I think I see where you could be coming from.
But I suspect you can only have sex with an indifferent partner a few times at most before you develop a gut-level rejection of having sex with an indifferent partner. At least, something like that has been my experience. Maybe some others have a greater tolerance for indifference, apathy, lack of interest. From what's been said in this thread, it would no longer surprise me if that was the case.