Homophobia.

Who's up for International Day Against Homophobia? (Monday, May 17th)
All you have to do is wear a plain black shirt to silently protest homophobia. If you can, you can write "Gay rights are human rights" on your arm or on a piece of clothing.
*definition* Homophobia: The fear of homosexual behavior and those who practice it, often manifested as hate.
And by behavior, I mean everything, including kissing, holding hands, and the simple act of loving. (I know, loving can be anything but simple. But falling in love can be easy.)
This might not be the best definition. But I think it's the closest. Remember that post about labels? I would wager that homophobia is mostly about fear of the unspoken 'rules' getting broken.
I think that transphobia takes a slightly different definition than homophobia, but for simplification, assume that trans-phobia is included in, but not the same as, homophobia.
Transphobia: The fear of behavior and dress as though one were the opposite sex (especially in M2F trans people), often manifested as hate.
Homophobia is a horrible thing. It makes people fear themselves, their friends, and their family in the name of 'what's right'.
It is not a disease to be GLBTQ. Be proud.