Because some people do not accept people who are different. Plus I think a lot of people just do not understand what it means to be gay.
You also need to understand that those who do not accept them are most likely coming from a time period where it was not acceptable.
For those who hate on gay people are different story. They HATE it out right and that hate makes them dangerous. They do not care about understanding people who are different. It is the hate that controls them. Just as those who hate people because of the color of their skin.
There are always going to be people who hate people who are different. I seriously doubt that will ever change.
Fear of those who are different & fear of those you don't understand has always been around. It isn't going away.
It is wrong to hate someone for any reason. It is wrong for people to hate someone over something that they can not control.