My sense is that yes, it's mostly gay, for obvious reasons.
For straight guys here, it's usually not about actually finding the other big dicks sexually attractive, but rather about using this site simply to show off or harmlessly brag about their assets (something that is obviously hard to do in real life without coming across as a real, ahem, prick).
And just so nobody gets too mad at me for saying that, I fully admit that that is often my motive, as well. There's nothing like posting a dick pic or an anecdote about shocking a nurse to harmlessly boost one's ego for a minute or two. Certainly beats flashing people in the subway ...