The joke is that in all the talk about gay and straight in both politics and the schoolyard the possibility that someone could be bisexual is often overlooked or belittled (how can you be gay if you also like that girl? is a literal sentence I've heard said to a friend of mine).

Hence bisexual people are often said to be "invisible", this joke has fun with efforts to raise awareness of bisexual people.

Whew, good thing you warned me…

I had no Idea.