I guess being a gay man, eating cum is kinda second nature to us. On one occasion I had a 3some with my str8 friend and his wife, I ate his cum out of his wife while he ploughed my arse. I love feltching from a guy too, but a number of conditions myst be met before this happens. He must have done a proper douche, there must be no farm-yard smells emanating from him after all an e-coli infection is just not nice.
I am a big fan of rimming, I actually prefer it to sucking a guy's cock. The anus has probably as many nerve endings as the head of a penis. Ive found that by rimming a guy for between 30 and 45 minutes, he'll start having anal orgasms. (google that). I often joke to people that i have a PhD in rimming.
Feltching is just a natural progression from rimming.
Straight men, please note that you can do this to women too. Think of her pleasure, not yours!