Why does Victor refuse to make a female monster?
Concerning Victor's refusal to make a female in Shelley's Frankenstein, actually, Victor eventually agrees to make a female. At first he refuses, but then he agrees. He travels to England to learn from experts there, then travels to Scotland and finds an isolated area to work. He has the female partially built, when
I trembled, and my heart failed within me; when, on looking up, I saw, by the light of the moon, the daemon at the casement.
The "monster" surprises Victor at the door of the cottage. Victor has a knee-jerk reaction to the look on the monster's face:
As I looked on him, his countenance expressed the utmost extent of malice and treachery. I thought with a sensation of madness on my promise of creating another like to him, and trembling with passion, tore to pieces the thing on which I was engaged.
So his "refusal" to make a female is actually a passion-filled gut reaction to the monster's facial expression. It's not reasoned out, but, instead, is an emotional reaction to the monster's surprise presence at his door.
To find the answer for this question, you can look in Chapter 20. Victor gives a number of reasons there.
He worries about making another monster because he wonders if the female monster will be just as bad as or even worse than the male monster.
The male monster had promised to leave Europe but the female hadn't. What if she refused.
What if seeing the female monster made the male more aware of his deformity and more angry?
What if she doesn't love him? Won't that make him angrier?
What if they reproduce and make a whole race of monsters?
When Victor Frankenstein created his creature, he was so devastated by the results that he had rejected him. The creature was left to fend for himself in a world that could not accept nor tolerate his looks. Victor was mentally plagued by his own anguish over what he had done. In addition, the presence of the creature was a constant source of fear for Victor. He lived his life wondering when the creature would appear and what he would do next.
When the creature asks and demands that Victor make him a mate he promises Victor that he will never see him again. Victor by that time had become more cognizant of his crimes against the creature and nature by having tried to make a living being without pre-analyzing the consequences for the creature or society. He refuses for this reason, and also because he thinks of the idea of the creature procreating and creating a race of such beings; and that the creature may well return to society despite his promise. He knows the creature craves social interaction and does not believe he will really stay gone.
Frankenstein feels that he would be saving society by not creating a female companion for the Creature, because the creature and his female mate could procreate and in doing so create an entire "race of devils" which would destruct the world.
"you will then have a companion to aid you in the task of destructions", Frankenstein is aware that the Creature feels the need to revenge on mankind for being constantly rejected by them and treated like a monster. This likens the creature to Satan, as the Creature has previously described satan as having "devilish companions".
So overall, he fears that his creation will cause horror across the world, because he made the creature bigger and more powerful than humans, and if the Creature and his companions procreate then they will be a great threat to all of mankind.