When it comes to porn search engines, I think that the biggest daddy in the arena is Porn MD. See, this place has just gone leaps and bounds ahead of a lot of the so-called 'competition' to provide a great porn viewing experience. Oh, it's also managed by the guys behind Porn Hub and Gay Porn too - they have quite the experience in the adult field.
Anyway, when you head on over, you'll see a really simple search interface with the ability for you to type in keywords and select from either straight, gay or shemale material. Sadly, these aren't check boxes, so if you consider yourself to be bisexual, you're not going to get the best of both worlds.
Anyway, when you type in a keyword and hit 'search', the self-proclaimed world's largest search engine will show you hot videos from various tubes around the world and allow advanced filtering based on length, HD status and when it was uploaded. Check it out, basically: this place has some killer ability to give you top level gay porn search results.