Pretty self-explanatory. A flannelgay is an endearing term used to describe a bro who digs wearing flannel and doesn't hide his appreciation of other bros who also like wearing flannel shirts.
Oh dude, I had no idea you also loved wearingflannel. In fact, we are matching and that makes us flannelgays.
I was all ready to hunker down for a long evening with gimp fetish magazine, but my ejaculation flannel was nowhere to be found! blast rag spankerchief