Is ash's Pikachu a boy or a girl?

ash's pikachu is a male because the females have a dent on their tail and pikachu doesn't have a dent so it proves pikachu is a male

No,Pikachu is a male,but not by the reason above.Pikachu was introduced BEFORE gender differences actually existed,so it´s not proof at all.

The proof is in "The Evil Togepi" episode,where a Togepi uses the move Attract,which only works on the opposite gender.

Pikachu,Piplup,Meowth and Croagunk are affected,but Jessies Yanmega is not.Jessie wonders why this happens and Yanmega herself says that´s she´s female.If Attract only works on the opposite gender and did not work on Yanmega because she´s a girl,then the Togepi in question was female too.Attract worked on the other four Pokemon,including Pikachu,therefore confirming him as male.