There are still people who seem to think it's about sex. Maybe for some people it is but usually when people complain about not having any sex, it's because no-one will have sex with them, not just because one individual won't. They might talk about how they are always being friend zoned or something along those lines.
What I think most people mean when they say that they have been friend zoned (it's what it's always meant between me and my acquaintances anyway), is that you feel romantic feelings towards a friend who doesn't feel the same way. There is a specific target of their affections, someone they develop strong feelings for that go beyond feelings of lust. They may be already having sex in a FWB situation as I was and so fulfilling their physical desires, but want a deeper emotional connection that you can't get from just sex. That desire to connect with a particular person in a deep way that goes beyond friendship is, to me anyway, much stronger than lust. If I have a friend I'd quite like to have sex with who doesn't want to have sex with me, which has happened relatively often, that's fine, it might be a little ego bruising but I get over it, there will be other chances for sex. When you fall for someone head over heels, someone you're just desperate to spend time with because you love their company, someone you'd willingly not have sex with if you thought it might damage your relationship, and they don't feel the same about you, they just want to be friends, that's the friend zone. And it hurts. Unlike being denied sex, where there are plenty of other potential options, there is no one else who makes you feel that way. And this feeling can only develop between friends, because you can only develop feelings that strong about someone you know well and spend plenty of time with. And sometimes, spending more time with that person just makes the feelings stronger and the only way to get over it is to distance yourself from the person for a time.
Are there really lots of people out there who have relationships based purely on sex? Have the people who think it's all about sex never been in love, never had the feeling of missing someone so much it makes them feel sick, never had someone they could just talk to all day about nothing and then cuddle up and watch a movie and not have sex and it be the most awesome day ever?
(I may feel so strongly about this because I'm missing my fiancée who lives half way across the world from me at the moment and we left each other a couple of weeks ago. If our relationship was only about sex, we wouldn't be able to cope with only seeing each other 6 weeks in a year, 5 months apart. We had sex before we even knew each other very well. It wasn't until we really got to know each other that we started feeling strongly for one another. And when her visa ran out, I friend zoned her because I couldn't see myself in an LDR. We still saw each other semi-regularly for a bit and were still having sex. Again, the friend zoning had nothing to do with her desire for sex. She's getting just as little sex now and she's definitely not in the friend zone. Then a couple of years later I came to my senses and now we're together).
TLDR: I think for most people, and definitely for me, the feeling of rejection you get when your rejected sex sucks but is no big deal. That isn't the friend zone. The friend zone is when you fall for a good friend and they reject you. Call it unrequited love if you prefer but it's the same thing.