Somehow it turned into one of the more active gay porn subs, so there are actually a lot of gay and bi guys there.

But in terms of the "straight" thing, I think there is a complicated mixture of stuff going on. There isn't really universal agreement about how sexual orientation should be defined and categorized, largely because non-heterosexual orientations are still to some extent taboo. For the most part, we define sexual orientation in terms of who we are attracted to, but for some people it's about who we have relationships with, or something else. So some men who are attracted to both men and women consider themselves straight because they are married to a woman, for example.

In addition, there are lots of people who desperately cling to the straight identity for some time while they are coming to terms with being gay/bi. When I was growing up, everything I heard about gay people was extremely negative, so I didn't think of myself as "gay" for a long time after I had accepted that I was only attracted to other boys. And then some gay guys fetishize straight guys in the same way that straight guys fetishize lesbians (or the same way that you fetishize gay guys :P), so some people who hang out at the sub just like the idea of straight guys being into gay porn. And also some straight guys might fetishize the idea of gay sex without really being interested in trying it, in the same way that some people might watch BDSM videos but hate the idea of actually trying it irl.

Some of these reasons also help to explain why it's so hard to measure the proportion of the population who are gay or bi, and why slightly different surveys can obtain very different results.