I have them from time to time.
When I was younger, typically I'd have one after the gf du jour and I had been dry-humping or something, but hadn't actually fucked. I'd wake up in the morning with a hardon (I most always wake up with an erection, so that part wasn't unusual), and a big mess on my chest or the sheets.
Nowadays, since I'm getting laid more often, I get 'em when I've been having daily sex, then stop -- like if my girlfriend has to take a business trip out of town. The first night we're not sleeping together, typically I'll be waked mid-night with a hardon and semen-soaked sheets. If she's only going to gone for a few days, I'll "save up" semen for her return. After the first night, usually no wet dreams. Then, good sex when she gets home.