Nocturnal emissions are basically orgasms without direct stimulation. I fondly remember these from puberty that is when guys start having these. At this time, a man is producing copious amount of testosterone as well as sperm and semen. Without sexual release, this combination will produce a wet dream.
It is not unusual for older men to have wet dreams as well if they don't have regular sexual release.
BTW, women have orgasms when they sleep too.