As a gay woman, I don't believe it is intrinsically discriminatory.
Often the discussion of LGBT topics includes discussion on sexual orientation. The Hubpages policy/automatic filter grabs on to the sexual parts of the comment. As we know, machines can't infer the tone or topic of conversation, so it sees "sex" and goes "ads off"
It is noted that ads are disabled on both pro and anti gay hubs - when a policy is following for both sides - it's fair and equal.
I know when I write on LGBT topics, I may not have ads - but if it's important to me, I'll still post it.
THe thing is is that it's actually out of HP's hands anyway - it's often the Google Adsense rules or others that force their hand.