Such things happen to everyone (well, males, of course). It doesn't mean you're bi or gay or anything you don't want to be. The average man gets an erection 11 times a day, randomly, for no reason. Yes I said 11. Males, in the cave people days, did the hibbidy dibbidy all day long, or whenever they felt the urge. Males of this day in age are just damned horny all the time because of it. Don't worry about it. It also tends to happen less when you are more sexually active.
._- g e i k -_.