[elisbce]

That's just because you separated the less sexually attractive ones (to you or vice versa) into the "friend zone" and you don't want to mix them with the ones who you might want to have a sexual relationship with.

But fundamentally, friendship (getting along, having similar tastes and views, etc.) can be viewed almost orthogonal to sexual attractiveness. So there is a group of people who are both friend material and sex partner material. You have to deliberately suppress your natural feelings of having a friendship AND having a sexual relationship, and make a choice due to fear of social norms. You might be able to do that, but the desire is still there. That's why there is no real friendship for a lot of people because it is hard to ignore the sexual desire. A moment, a spark can lead to a night of romance between two close friends who are sexually attracted to each other. And this whole friendship thing could collapse.