“Isn’t that the defining difference between a friend and a potential lover?” No; romantic relationships are more than just sexual friendships. The defining difference between a lover and a friend is that you (eventually) share an entire life with your lover. Lovers should become far more important to each other than any friend can be, which wouldn’t make sense if attraction were the only difference. You should love your partner enough to always exclusively put them first–as if they are the most important half of your own self–which also wouldn’t make sense if attraction were the only difference, and wouldn’t be possible if anyone else were equal to them because it’d create a conflict of interests. Assuming you have no children, of course.
Lovers depend on each other more than friends should, because as life partners, any decision either of you makes affects the other. This also gives you some measure of rightful authority over each other. Many couples even lose their attraction over time, or their attraction comes and goes, but the fundamental nature of their dynamic doesn’t change. The emotional/psychological aspects of their relationship outgrow the physical aspects and become far more important to them. There are more boundaries between friends than there are between lovers, because friends live almost entirely separate lives. Even if two friends tried to “live as a sexless couple”, this would prevent them from forming full romantic relationships with others; they would essentially become a married couple that just never has sex.
Why do you think losing a lover typically hurts so much more than losing a friend does? Again, this wouldn’t make sense if attraction were the only difference.
Physical attraction can even stem from mental/emotional attraction, yet most people who have experienced this have never lusted after our closest friends. Even “demisexuals” rarely lust for our friends. This further proves that the psychological bond between lovers differs from that between friends, and why we should open our minds to falling in love with people we aren’t physically attracted to. Getting to know the person even better may change the way you feel about their appearance. Falling in love with “ugly” men has made them less ugly to me; I still saw the way they look, but their images came to represent something special to me. Humans find certain things “beautiful” and “ugly” because of their positive or negative associations. Seeing those pimpled faces and scrawny bodies excited me because I knew they were attached to someone who makes me feel good in the most special ways.
We cannot always choose our tastes, but we can always choose to look beyond the surface. If you’re going to reject someone, let it be for lack of mental/emotional capability, then you can rightfully “friendzone” them.