Earlier quoted context omitted.
This is baseless, unscientific blank-slateism, and isn't true anywhere in the world among any demographic segment in any measure. If 10% of Singaporean men are gay would you expect 10% of male plumbers in Singapore to be gay? If women are 30% of the workforce in Canada, would you expect Canadian dental assistants to be 70% male? Disparate outcomes doesn't imply unfair treatment.
"given no structural impediments" is the keyword - without any structural impediments, all your examples should be true. Structural impediments aren't inherently unfair - for example, if gay men simply prefer not to be a plumber for some unknown reason, that is enough to say that there is a structural impediment for gay Singoporean plumbers, and there doesn't have to be anything unfair about that. On the other hand,…
I can't imagine a good reason why young gay singoporeans wouldn't dream of being plumbers any more than the next kid.