I reentered the US yesterday at DFW airport with my girlfriend. We had traveled together the entire trip and approached the booth together, as we have done in the past. The officer was friendly about it, but said that in the future, since we aren't married, we have to come up separately. But he said this time he wouldn't send one of us back, so as not to "embarrass" us.

Is this a rule/law? I feel like it would be easier for the officer to talk to us both together, since we did the whole trip together, had purchases for the two of us, etc. And two single people traveling would probably make the officer more suspicious, and take longer, especially since we actually did travel as a couple. Is it really that different if you are technically married?