Ah, good question.
For issues of sexuality, look at the sections in the book directly or tangentially related to sex: Victor's marriage night and the creation of the monster are the two most striking examples. Rather than going to bed, Victor prowls around the house with his hand on his gun (a common symbol for a penis), and eventually finds that the monster has killed his new wife.

If you look at the creation of the monster, it reads like a sex scene. Victor thrashes around, passes out, and just about lights a cigarette when he's done. Then, the next morning…seriously. Look at those passages.

Gender…is a bit more subtle. I'd start with the male dominance in the novel: male creator, male narrator, etc. Look as well at Victor's blindness regarding creating a mate for the monster, and the monster's desire.