Is it normal to only think about sex with a boy you don't know but like?

There's a coworker I like. I've only known him for a month and we've never really had a conversation. I feel somewhat attracted to him, so I assume I like him. Anyways, all I think about when I see him is him doing me! I wake up thinking about this, sleep thinking about him, its all the time! And its not just something like 5 seconds, its vividly and very detailed, and last hours! Anyways, is this normal?