Ya, it is mostly so your junk doesn't get squished or sweaty. But there are other reasons too. Men are generally taller than women (by at least 5 inches average). If you are sitting on a side seat, It is hard to keep balance because of the higher center of gravity when the bus starts and stops. It seems like most of these "man-spreading" pictures are taken of men in these side seats. Also men are generally bigger and have longer legs because of the height. Often you have two options when you are tall, keep legs together, have uncomfortable balls and block the aisle or spread them and take up more space to the side.