That's a fair question, and I think there's a website where you can run down who supports what. It was my impression that NGO's like GLSEN and its parents get their money from foundation grants mostly, and from contributions by gays.
The amount of liberal-controlled foundation money is like the Mississippi River, comparable to the budgets of civilized governments -- it's huge. I stumbled across the budget of a liberal foundation out West that I'd never heard of before and began reading what they were paying out and to whom (unilateral disarmament seemed to be their "thing," that and spreading negative PR for companies that worked on weapons programs), and I was amazed and appalled. These guys had their hands on real money and were handing it out like quarters in a videogame arcade to Left NGO's.
And they complain about Richard Mellon Scaife.
I think school districts buy all the materials. If GLSEN isn't profiting directly, then they are indirectly by promoting the materials of their own donors.