I had a strange experience with this. Two times ago when my then boyfriend and I travelled together, we went through the seperate lines. The IO asked me if I was travelling with anyone and I said my bf and that he was in the EU line. She said "Oh, go with him through the EU line next time. As long as you are travelling together, you can use that line." I was surprised and she explained, "it keeps the lines moving faster if we see travelling parties together and as long as he's with you, you'll get through faster."
So.... the next time we were entering, we went through the EU line together. And asked the IO at the desk if that was okay. He said "Of course. But you need me to stamp your passport don't you?" I said yes. He did not have the stamp but his colleague a couple of stands down did. He said, "Go to her window and she'll stamp you." He called down to her while we were walking over. She said "Are you married?" We said no. She then said I could only go through the with my boyfriend if we were married. She then sent us back to the "other passports" line where I got to wait all over again.
Interestingly, as we walked back by the original IO he looked confused. When I eventually reached the IO in the "other passports" line, I asked about what we had done. The IO said, "The hard rule is that you are supposed to be married. Do we enforce that? No. You were absolutely fine to go through the EU line with your boyfriend and I apologize that my colleague wouldn't stamp you in." Ha!
So, the hard rule is that you are supposed to be married but that it is generally accepted and encouraged to go through the line together... but you might get that one IO who sends you back to the other line (even if the person controlling the lines says you are fine).
