1girl 1boy are pointless tags. We already have "female" "male" and "solo". So either we get rid of "female" "male" and "solo", or we don't use "1girl" "1boy".
These aren't replacements or supplements for those tags. Unlike solo or -male/-female, these tags aren't exclusive of other genders, so they can find specific gender ratios. An example Hinacle mentioned earlier in the thread being a non-couple related image with 1boy and 1girl.