You are on to something about the IQ comment. If we look at past rumours, players have been identified as "potentially gay" for reasons such as reading books, being interested in art or having any hobby that isn't playing FIFA or whatever they do. Picture thinking about coming out, and then you see a married team mate being ridiculed as "probably gay" because he knows how to find the entrance to the national gallery. That doesn't shout "it's safe to be who you are".It’s weird isn’t it, and I can imagine the agents must play a part.
You also have so many different cultural backgrounds, and of course regular close proximity in the changing rooms. Plus without meaning to generalise too much, footballers usually aren’t the smartest bunch.
What surprises me is the lack of former players coming out.