Siem Reap is a great place to visit.
Of course they gay scene isn't as large as Pattaya, however the temples are amazing and the town has a nice atmosphere.
My fist ever trip to Thailand included a side trip to Siem Reap and I've been back to that town 3 times since.

You wouldn't go there specifically for the gay scene, however visiting all the temples makes a great way to spend 3 or 4 days. Then in the evening, there are plenty of good dining options in close proximity to each other and some gay options for afters.