* The assumption that all characters are heterosexual. This is evident in the way that the characters talk about love and relationships, and in the way that they interact with each other. For example, when Viola disguises herself as a man, she is immediately assumed to be in love with Orsino, even though she has never met him.
* The portrayal of same-sex desire as unnatural or comic. This is most clearly seen in the character of Sir Andrew Aguecheek, who is constantly making jokes about his attraction to men. His desire is not taken seriously by anyone, and he is often ridiculed for it.
* The fact that the play ends with a traditional heterosexual marriage. This reinforces the idea that heterosexuality is the only acceptable form of sexual orientation.
The expression of heteronormativity in Twelfth Night is problematic because it marginalizes and excludes non-heterosexual people. It also creates a climate in which it is difficult for non-heterosexual people to be open about their sexual orientation.