At Gabriel's behest, I am providing an illustrative example of the difference between a close platonic friendship with someone of the opposite sex, and a romatic relationship, and how that difference is not mrely about sex.

Platonic friendship
a) You sleep in the same bed as your close platonic friend, but you never sleep with each other
b) Your close platonic friend pimps out her female friends for you, and you likewise bring out your male friends to pimp for her. (assuming both parties are unattached)
c) You call each other "bitch" and "asshole" repeatedly and you under no qualms about telling each other to fuck off without rancour if you are tired of their presence or if you have better things to do.
d) At clubs you dance close to each other but never with each other
e) Even when drunk you don't try to kiss each other or feel each other up

Romantic relationship
a) You sleep in the same bed as your gf, but you don't sleep with each other because she feels she's nto ready to take the next step
b) She worries about you hanging out with female friends; you worry about her hanging out with male friends
c) You call each other "darling" and "honeybunny" repeatedly and both of you take pains to hang out or see each other or do things for each other despite having better/more important things to do
d) At clubs you dance with each other
e) When drunk you both start crying melodramatically about loving each other and only wanting no one but each other and helping each other clean up vomit stains.


Postscript:

Gabriel: your post is not what I was looking for

so I assume the difference is merely about sex
hurr hurr

HWMNBN: no
there is no sex in both scenarios
it's about expected behaviours more than sex

granted i chose stereotypes
but we've had the stereotype conversatio before

gah
if yo udon't like it go write one yourself

Gabriel: *facepalm*
there's a reason why I asked you to write it

anyone reading this can go: "oh yes, it's just about sex"

HWMNBN: NO IT'S NOT
READ IT CAREFULLY
IT'S ABOUT THE WAY YOU'RE EXPECTE TO BEHAVE WITH AND AROUDN EACH OTHER IF YOU'RE IN A ORMANTIC RELATIONSHIP VS THAT OF A PLATONIC RELATIONSHIP

above and beyond sex
i tell *** to fuck off all the time and i don't feel a need to constantly entertain her patter and vice-versa
whereas with *** it was pretty much a "if i want to talk to you, you have to talk to me" kidn of thing

i admit that relationships hafe many shades and nuances

Gabriel: that's because you want sex

HWMNBN: *hollow laugh* how much sex do you think i had with ***?

*GNASNGASNFHDFHNADNFHDSFH*S

although i will admit that sex is a huge part of the difference
one of the large rparts

but frnakly relationships have many shades and nuances
so there's considerable overlap between the two concepts depending on your interpretation
sex is the easiest dividing line to point to, but it's nto the only difference