The same thing is always happening to me. I just go out for a drink with my friends and return home with wet dreams of some unknown girl I met night before. What is wrong or OK with me? Is this normal? What is that this girls has that turns you on in a chick you don't know?
That simple fact that you do not know some girl is completely intriguing by itself. That fact makes you curious and interested in her. And your hormones, off course. Testosterone is made for this kind of stuffs. Next picture in your head-you and her together. The rest of the scene imagine by yourself.
If you find a girl interesting in her outer beauty, you need to go talk to her...you may find that she is not the raving beauty you think, as the saying goes, "Beauty is only skin deep" But find out what is inside that makes her.. who she is from the inside out. You may encounter an awesome relationship with one. And rather than just having wet dreams, you may find a true and meaningful relationship.