You may have to come to terms with the thought that the only reason some of them were ever friends with you is because they eventually wanted to sleep with you. Realize also that this reflects on their character, not necessarily on yours.
But unless you're seriously smokin' hot, that probably only accounts for a small number of them. An alternative is much more likely. Even guys who have an earnest interest in you as a person may not even realize that there is a boundary to accept- that there isn't any difference from liking them as a friend and liking them as a boyfriend, and that the latter is just a greater degree of the former, not a different category entirely. That's certainly the way most men feel about women, myself included.
You see this all the time in relationships: two people that have had a healthy platonic relationship seek a deeper one, and then things become 'complicated,' usually because the guy is still acting like just a very good friend, to the consternation of the girl.... who has entered full-on relationship mode, to the consternation of the guy.
This is why women think their men don't care and don't want to commit, and why men think their women have changed overnight and gone insane.
So when you tell them you don't want to be their girlfriend, to you you're telling them that their just not in that category. To them, you're saying you don't value them as much as they value you. And, intentional or not, that's the kind of stigma that can quickly destroy any relationship.
So please don't think this is just a case of guys being shallow or insensitive; it's likely the opposite that is true. Once one of your friends start to feel that they've been 'rejected' by you, they'll start questioning what it is about themselves that you saw as unsuitable or inferior, or they'll feel that you're only friends with them for your own sake, not for the sake of both of you. And in a world that only values men for their competence and strength, no man will stand for being made to feel inferior, or for being used. If we think this is the case, true or not, we're out the door.
I really don't have any advice for what you should do differently or how to evoke a different response. That depends on your specific relationships with your specific friends. Hopefully, understanding how they feel a little more clearly already does most of that work for you. But even knowing that it is very, very difficult to overcome these reflexes- even just coming out and saying directly that these things aren't the case will usually elicit the feeling that you're covering for yourself before the fact, which adds the added feeling of being lied to. And the reason it is difficult to avoid these kinds of responses is because most guys don't understand them themselves. And if you tried explaining them, they'd think you thought they were stupid and shallow, sparing neither of you any grief.
If all else fails, wait for a light rain and hold a boombox above your head outside their window. If that doesn't work, nothing will.
