Why is BDSM thought of as bad or gross by a lot of people?

Because it's a fetish, and therefore constitutes private personal information that others don't want to know about. People generally don't want to hear about what gets someone off - especially when what gets them off is commonly associated with whips, ball gags, and leather bondage.

Very few fetishes are able to be discussed openly without causing a negative reaction.

It is a part of the hidden programming that controls human behavior. Part of the programming is to organize into tribes. A family can be a tribe, so can a sports club, church group, political party, nation, etc. The program will reject those that do not believe in or practice the tenets of the tribe. And so BDSM is rejected by believers in egalitarianism and nonviolence, and most especially by those that do not understand masochists. It takes a masochist to really understand a masochist. All others just do not understand.

Adam Leonard , "Man By Nature: The Hidden Programming Controlling Human Behavior"

http://www.amazon.com/s/?_encoding=UTF8&camp=1789&...

Because a lot of people are insecure, jealous, not confident or immature - even adults. A guy and a girl who are involved in a BDSM relationship must be secure with their sexuality. They need to be comfortable being a man or woman - and enjoy it.

Some bad stories can come from people into BDSM. That's probably why.

I think its self explainatory. You see someone who looks like they are being abused or raped and that's a red flag. Its easy to see how some people think that its creepy to be interested in.