Question

why emasculation is not required in unisexual flower

Answer 1

because emasculation do for stop or prevent self pollination.. and unisexual flowers have only one type of gamete hence it is necessary to prevent self pollination in bisexual floers

Answer 2

Emasculation is not required for unisexual flowers because emasculation is done to stop self-pollination within the same flower.

Emasculation is a process in which anthers of a plant are removed or pollen of the flower is killed so that it does not self-pollinate with its same female reproductive organ.

In the case of unisexual plant a male flower will anyways not have a female reproductive organ thereby preventing self-pollination between flowers.