I went there in 2010 and found the only place for any pick ups was the Linga Bar.
I stayed at the Mens Resort which had lots of cute staff but none were actually gay.
There is a gay sauna at the Mens Resort and a few locals visit but depends what you are after.
I found a good selection of guys at the Linga who come up and chat to you and are all willing to come back to your hotel.
I gave them $25 and had no complaints.
I did not see (or want, I hasten to add) any underage and all hotels ask to see the id of any guest you bring back..
Not sure what the scene is like now though but if you combine your social life with the beauty of the temples at Siem Reap you can have the best of both worlds...