Here's what I've heard about "curses". If someone wants to curse you, they have to let you know. Like, if you tick Bob off and he wants to get back at you with a curse, he can't just say or think to himself, "I curse Jim with never being able to get it up again". He has to tell Jim, "Jim, I curse you with never being able to get it up again". If this isn't true, someone let me know.
Now, this being the case, a curse is just an exercise in the power of suggestion. It's in the cursee's head. Jim may think, "Whatever. Bob's crazy!" But subconsiously, Jim's mind has already taken this info in. Next thing you know, Jim can't get it up.