Afraid I might be repeating myself - but here goes, anyway:
I don't think fewer boxes would be opened if we knew the gender beforehand. -In my cattery, I got lots of boxes standing around, waiting for me to be courageous enough to dare the GENDER-LOTTERY... which only happens on a VERY good day. Some of these boxes have ended up standing around for SO long that I long surpassed that "level" of breeding... and then I clean out, sending them all to menagerie with a sad sigh.
These are kittens that I would have fed and bred if I had known beforehand that I could - that I wouldn't end up with 2 girls, 2 boys, or just the wrong gender to breed back, if that was my plan.
Also, I would feed and breed certain pairs much more joyfully, saving the boxes they give untill I had one of each gender, and then happily unbox those and continue the breeding. -In trait-combination-breeding, this would help SO much.
Apart from that, it would, as some have already stated, help the secondary market a LOT to see gender on boxes - as it is currently, boxes just don't sell. You HAVE to unbox whatever you hope to sell, and then have to feed it untill it sells - and when some days or even weeks pass without sales, that is when the fire-sales begin.
-And THEN we see people giving up on KittyCats alltogether, because most breeders have NO chance of earning even a faction of the food-cost back again.
In my view, KittyCats could have a much bigger market-share in breedables if it wasn't for 3 things - gender on boxes being one, the time a kitten needs to grow up being the other, and the total cost of food+milk being the last. -But these two last ones are different discussions for another day