We don't have these two tags currently, but I think we should have them. My reasoning is simply: as far as I'm aware we don't currently have a good way of tagging an image with one guy and one girl in the image, unless they're a couple.
Additionally, this tag can be used on images that have one guy but multiple girls, and vice versa. Also, solo is used both for images with one guy and images with one girl, so it provides more granularity for that too. It is simply information that can be tagged and is useful for certain searches, and is currently falling between the cracks, untagged.
I think we can start populating this tag by doing a mass edit of couple -> 1boy, 1girl, and possibly going through the solo tag (manually, since that includes both girls and boys). After that it's just a matter of using it properly from now on out. We already tag 2girls and 2boys etc., so I don't think it'll be difficult to learn this behavior.
I don't know if this has been discussed before (I imagine so), but I'm curious to hear arguments for and against this.
EDIT: Just to clarify, I think we should use these tags on EVERY image that qualifies. So regardless of whether it's a couple, or a sexual situation, if you see one girl in an image, tag 1girl. Analogous to 2girls etc.
Updated by Saladofstones