well don't ask feminist organizations, ask the plumbers and the customers:
https://www.workiz.com/blog/plumbing/why-how-hire-women-plum...
The second reason comes down to client safety. For various reasons, some women simply aren’t comfortable with male plumbers. Though most plumbers are perfectly nice people unless you’re a particularly spirited clog in a drain, the fact remains that not every woman is comfortable being alone in her home with a man that she doesn’t know. Having access to industry professionals that can provide a little more comfort can be a huge selling point, particularly for young women, single mothers, and victims of domestic violence.
https://www.worldplumbing.org/attracting-more-women-to-plumb...
https://www.commusoft.us/blog/why-the-industry-needs-more-fe...
“We work with women that are survivors of domestic abuse. At a women’s safe house, if some work needs doing and they need to contact a tradesman, it can often be quite stressful for them. We get inundated calls from customers who want to use tradeswomen.”
so it appears there is an objective need for female plumbers, not just as a statistic, not just to create a better workplace, or higher shareholder returns. those arguments are given too. but with single women being more common and higher awareness of domestic abuse, the argument can be made that with 30% of women being single, 5% female plumbers is not enough.
this is a very rough estimate. 30% single women is 15% of the total population, but 23% of the total number of households. (30% single women + 30% single men + 70% each of married men and women adds up to 130 household units)
however if you add 25% stay at home women who are usually the ones who would deal with a plumber since the husband is at work, then they will probably prefer female plumbers too. so i'd estimate that we need at least 25% to 30% female plumbers.
it can be argued that being served by a tradesperson of your own gender is a right, and therefore a gender distribution that matches the needs of the population is not just something that would improve the trade (which can be argued about), but actually a necessity that should be enforced by law.
same goes for teachers btw. boys need male role models, and therefore i believe laws should push for a 50/50 gender parity among teaching staff at all levels.
similar arguments can be made for all consumer facing professions. (police for example). this is no longer a question of qualification or job preference. it's not even a question of gender equality, but a question of how to best serve the population as a whole.