..think it could be the testosterone??
It could be pheromones, a subtle sexual scent that animals exude. A gym where men perspire is soaked with it. You won't necessarily be conscious of it, humans usually aren't, but your olfactory cells will be triggered nevertheless, causing a sexual response.
Straight females respond to male pheromones, and straight males to female pheromones. But research is suggesting that gays & lesbians are attracted to same-sex scents.
Which of course would imply that sexual orientation isn't a choice, but innate, since we can't choose to react to scents that we are unable to detect on a conscious level, versus the way we can recognize the distinctive smell of a rose, for instance.
The first time I was taken to a crowded gay night club in Seattle I was hit with an incredible sexual rush. I almost got light headed, and no, there wasn't marijuana in the air. I still get a little buzz today in the packed indoor company of many other men. I strongly suspect I'm reacting to pheromones, and you may be, too.
In which case, enjoy your undoubted sexual orientation. If the resultant hardon is a problem then wear a snug jock, or compression shorts. But if it's a pheromone reaction there may not be too much you can do mentally to turn it off, because it isn't caused solely by what you're seeing or thinking, but by what you're subconsciously smelling.