"Let's Just Be Friends"
Possibly the most hurtful thing that you can say to someone who's in love with you, on a par with telling a guy that his penis is far too small to satisfy you, or telling a woman that her pussy stinks of rotting fish. It often means, and will always be interpreted as, "The idea of sex with you repels me".
It may or may not mean that the person genuinely wants to be your friend.
If you are ever in a situation where you're tempted to say this to someone, a far kinder alternative might be to say something like: "I'm really sorry, but although I really enjoy your company, I don't think we (are / would be) sexually compatible".
Some typical reasons why I do not want to Just Be Friends with you:
(0) I'm not a masochist.
(1) I already have plenty of friends, none of whom leave me feeling miserable, angry and sexually frustrated.
(2) There is no worse feeling in the world than watching someone you are in love with picking up someone else.
(3) I'll get over you a lot faster if I don't see you, hear you, smell you or touch you.
(4) I don't want to even think about you having sex with other people, much less hear the intimate details.
(5) I don't want to see you falling in love with someone else.
(6) I don't want to hear you crying about how the person you chose over me mistreats you, in the full knowledge that you won't leave them unless you meet someone just as bad.
(7) I'm not interested having all the responsibilities and none of the privileges of being in a relationship with you.
(8) I don't want to be your drink machine / meal ticket.
(9) I don't want people who *are* sexually interested in me being scared off because they think that you and I are a couple.
(10) I don't want you deliberately scaring off people who are sexually interested in me because you're jealous about me.