I am pretty sure I was no more than seven, and quite possibly two or even three years younger (this would be somewhere in the late 1960s, anyway), when I asked my mother why two men or two women couldn't get married. I am fairly sure I was just piecing together patterns about how people work, what it meant to be "married," etc., in an entirely age-appropriate way that didn't happen to be related to sexuality (though that's not why it was age-appropriate, if you see what I mean). She wasn't fazed by the question at all. (She may have given me a GLBT-erasing answer about the potential for having babies, but she didn't say anything openly hateful, and we did have much more nuanced conversations about sexual orientation later on.)
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