Head on over to r/gaymenfuck (that is, if it's males we're talking about here) and there's plenty of amateur good content there

You are right.  After a few drinks I may hint at it.  
I asked a different couple, they laughed so hard they were crying.  They referred me to pornhub.

Um, how about respect their privacy? Just because they're gay doesn't mean they're a novelty act for others' enjoyment. Then again unless they're into that then just say you'd like to watch sometime and that it'll get you off.

Offer to be the towel boy, or perhaps work the camera.

Just say something like "I'm sure you two fucking is quite the sight to see" or "I'd love to be s fly on your bedroom wall next time you go at it" and see how they react.

I want to watch a gay couple (that I know) have sex, how should I ask to watch?