>>I have never seen discussions about gender-based discrimination for plumbers

In general only jobs which have a high value of effort/money-earned are preferable to anybody.

Im pretty sure most plumbers don't get into plumbing because they are passionate about it.

Also curiously you can be passionate about something if you earn well doing it. So, ultimately its all about home much money you can make, at what effort.

Make plumbing a high paying job and calls for diversity in plumbing will go up like no tomorrow.