There are several regulations under HASAWA and elsewhere that restrict or prohibit unisex toilets in workplaces and public buildings, including entertainment venues. Properly managed unisex toilets make possible more flexible use of facilities.
Why does this idea matter?
In public buildings the toilet facilities for the audience are regulated mostly by building and licensing regulations, and often leads to long queues at one gender’s toilet while another is under used. For example Wembley Arena stages heavy metal concerts (predominantly male audience) and ice dancing (mostly female audience) the Gents has queues at the first, the Ladies at the second. Unisex toilets would mean less queuing.
In the workplace, the major regulatory barrier to installing unisex toilets is the Health & Safety Management Regulations. Unisex toilets in the workplace are not uncommon in other countries. (Remember Ali Mc Beal?) and if properly managed can provide better facilities for all concerned.
A barrier to increasing diversity in areas traditionally dominated by one gender is lack of facilities for the minority gender. This could easily be solved by introducing properly managed unisex facilities.
In older buildings with male and female toilets on alternate floors, conversion to unisex facilities can put local (and more accessible) toilets closer to everyone. In buildings with male and female toilets on every floor, unisex toilets can reduce the amount of space taken up by toilets, or make more generous facilities available to everyone.