Board Thread:Questions and Answers/@comment-49.145.133.249-20170712050427/@comment-86.4.53.237-20170712110147

The reason that's given in Apocrypha is that Victor Frankenstein wanted to recreate Adam & Eve. But because of man's limitations, he first built Eve (who is Frankenstein's Monster) so she could give birth to Adam.

It's kind of messed up, but it does explain why she's a girl here.