If everyone was a fireman, then we'd all starve to death for want of farmers. And yet ...
Heh...it was sort of a tongue in cheek comment.
Your comparison is not quite the same. Everyone is free to pursue a career in fire fighting and the most qualified will make the cut. This is not really the same as sexual orientation. If everyone was heterosexual then we carry on as usual where as if everyone was gay then we don't carry on as usual.
One of the factors that I use to assess the acceptability of some given behaviour is whether or not everybody else could also behave the same way. Is that a flawed consideration?
What does Pace Kant mean?