What is the gender standard of a serial connection?

Both. You need a male and a female end to couple.

Seems that all computer's serial ports are male, but it is ambiguous the peripheral gender...

That is common, but by no means a rule. It is not a standard per se, just more of a convention. In computers, like everything else, people are really resistant to change things that have been around for a while if they work well enough.

I see printers with female DB9 connectort and also male DB9 connector.

And that is why gender-changers, -couplers, and -adapters exist. There is nothing in the format itself that requires it to be male or female, ultimately “it’s all pipes”… well wires.