My High School GF's family kept a strict kosher kitchen. She generally kept kosher outside her home, too, but wasn't as strict as her parents. If she were to do anything to disrupt the family's kosher home kitchen, then she would destroy her entire family's kitchen. If she had a cheeseburger while out (she didn't, but used this example to explain the situation), she wouldn't be kosher for herself, but at least she didn't then screw up her whole family.
I don't think it's all that uncommon, I seem to remember the conversation came up a few times in my childhood/teenhood. Not saying I agree with it, just that it didn't seem so uncommon.
http://www.thejc.com/judaism/rabbi-i-have-a-problem/36263/is-it-hypocritical-keep-kosher-home-eat-non-kosher-out
Oh, and I suppose that dating me wasn't all that kosher, either.