I think this is at least partially it. It seems that software developers here in the US pretty much don’t care if one of their own sometimes takes it in the butt, but the situation might well be different in more staid white-collar jobs like accounting as well as some “macho” blue-collar jobs.
I’m seeing two different selection biases. One, certain professions may encourage or discourage gays from joining them based on perception. Two, certain professions may encourage or discourage a gay person that is already in them to be “out of the closet”, thus impacting the gathering of statistics.
Remember, at least in the US, being outwardly gay in the US Military was considered to be cause for discharge up until very recently, so it stands to reason that the number of obviously gay soldiers was less than the (unknown) number of soldiers who were actually gay inside but who didn’t show it. There’s probably going to be some lingering stigma in the service for a while, even after the rules have been abolished.
e.g. fashion
There’s a social convention that men should be dressed fairly plainly or functionally, and that dressing colorfully or just differently is somewhat effeminate. IME gay men are less likely to care about this convention, so they are freer in trying out new fashions. I think a similar psychology is at work with food, the theater etc.
One other thing contributing to the perception may be that people working in some artistic/creative fields possibly keep their on-the-clock and off-the-clock lives less separate. Some of the engineers in my company, outside my immediate team, might well be openly gay for all I know, but it is not relevant to their work persona.
If I were to take a guess, I’d think it’s primarily a social thing. In the same way the some jobs tend to have lots of men or women or a lot of a particular race or whatever, I don’t really see how gay is all that different. Obviously, there are some jobs that will have some selective pressure, like jobs that require a lot of physical strength, like construction, will tend to have more men.
I do think that artistic jobs tend to be more accepting, probably because of the creativity. I think jobs that aren’t as creative don’t so much have reason to consider or be exposed to new concepts and very well may lag behind social and cultural changes whereas arts will often be at the front of social and cultural changes. Or it could just be some level of confirmation bias, where its perceived that various artistic fields are more accepting of gays, so they may feel more compelled to persue those sorts of careers and it goes from there. Or maybe it fills some kind of neutral ground in the sense that a “manly man” will want to do something is physical or scientific or whatever and a “womanly woman” will go into things involving nuturing or social stuff or whatever, so those who see those concepts as anathema to themselves will be compelled away from it, and the arts just sort of catch some from both sides since it’s not either.
I’m not really sure just how accurate the idea the notion is though. Admittedly I don’t know very many personally, but I can’t recall anyone who I’ve known was gay and worked in the arts nor can I think of anyone that I know that works in the arts and I think may be gay. It very well could just a perception skewed by the movie and music industries which, obviously, aren’t going to be very representative of our culture at large.