I dunno, sometimes I think even that isn't enough.
If someone likes you, even if you tell them at the off that it's a just friends thing, they will accept it, but there's always going to be that bit of hope. or belief that willingness will change.
Way too many movies teaching guys that perseverance is the key. or that being friends could lead to love.......if you go by one movie, swapping your semen with her donated sperm, and inseminating that child yourself, will get you the girl.
Obviously not all guys though, some can and will respect that instant honesty and live with it.