But Thomas isn't really on the prowl all the time though
It doesn't matter if Thomas is always on the prowl, or never on the prowl. It has nothing to do with what his thoughts are -- it's his entire being. It's what makes him a vampire of the White Court, an incubus. The demon inside him resonates through every inch of Thomas' body, and everyone within a ten foot radius of Thomas will react to it, whether he's hunting them or not. You've read some of Harry's descriptions of what happens when Thomas walks in front of people when he isn't paying attention -- men, women, they all turn around and watch him, and follow him with their eyes. Thomas could be completely lost in his own thoughts and have a parade of people walking behind him because he can't even have a shot to turn down the "come hither, and make with the go-thither" (to borrow a turn of phrase from Harry).
Snowleopard and Hatshep are smack on the money with the term "predatory angel." Thomas is supposed to be so utterly divine in his looks and aura that only people who know what he is would be aware of themselves enough to back off. And in my eyes, Ryan Reynolds, though awesomely sexy, cannot pull it off.
Also, it's not just a matter of finding someone with the right looks to portray Thomas. In my opinion, you have to find someone who is willing to go into the same mindframe as the man. He loves life, he loves his parties, he loves Justine, he loves Harry -- he hates his father, but he's loyal to his sisters and would go to any lengths to protect them; he's a known layabout, but he's also a secret agent of the Venatori; he's vain, arrogant, charming and entirely too flippant, but he's also one of the only known White Court vampires that has known love, and feels guilt over what his Hunger can do to the people he's around.
You have to look for someone whose eyes will convey the daily circle of happiness and torture that Thomas goes through. For me, that person is Matt Bomer.