Why would your son and husband declare themselves female if they are not (I'm assuming they are not trans)? What a very stupid thing to do.
But they might feel female that day and why not. This is absolutely what this has come down to and this is the decision the Scottish government are entrusting to it's people. They don't want to collect factual data ie. a persons sex which is observed at birth, that will be useful and guide decision making, they to collect data that is based on how someone feels.
I really hope the people of Scotland just put whatever they like under this, after all the Scottish government clearly aren't interested in collecting meaningful data.