As in nobody making trans and/or gay characters for an Rp.
I feel similarly, though personally I tend to get more peeved by a lack of racial diversity. That's not to say that everyone should be attempting to play a race/gender/sexuality that isn't their first choice, but it does make me a little sad when everyone is a young, white, straight teenager, regardless of the setting. Admittedly, I default to characters near my age, or from cultures and groups that I'm most familiar with, so I do understand why it happens.