Everyone that is not on an alerted persons jury blacklist is potentially on a given jury. There are factors, like post count and hidden posts that will either put you nearer the front of the line or nearer the back of the line.

Hosts can also vote on threads. That did not happen in your case. That was a jury. No LGBT host would have hidden that thread, controversial as it may be to some.

EDIT TO ADD: People also cannot jury on threads they have posted in.