I've always had trouble with this. I'm pansexual which make things complicated in the first place, but I also develop almost unhealthily close relationships with my friends. This is causing serious complications in my current close friendship and I have no idea how to proceed. I'd like to date someone, but I could see my relationship with my friend posing a threat to any potential partners.
I used to assume that the line between friendship and romantic relationship was sex- but that's not quite right because asexual romantics exist. And what about demisexuals?
I've studied the triangular theory of love and I've made tons of Venn diagrams, but I still can't make sense of it. I suppose it could exist on a spectrum, but spectrums bother me.
I've heard that the line is kissing on the mouth, but that really depends upon the culture and the people involved. It's kind of like saying that the line between acquaintance and friend is hugging when there's obviously much more than that.
Sorry I don't have answers for you, but its nice to know I'm not the only one with this problem.