Of Twelfth Night by Shakespeare, Joseph Pequigney writes "Olivia ends up engaged to marry a perfect stranger, Sebastian, and not the one she fell madly in love with and thought she had become betrothed to, who all along had been a male-impersonating girl. If she misses the tell-tale signs of femaleness that Orsino picks up on, that is because it is in her erotic interest to fantasize Cesario as virile; yet the feminine subtext, however ignored, remains legible." What would you make of this marriage and why?

In the play, Olivia falls in love with Cesario (who is really Viola in disguise). She rejects Duke Orsino even though he pines away for her. Olivia eventually marries Sebastian, Viola's twin brother, but not before she realizes her mistake. However, we soon see that things have a way of resolving themselves in Shakespeare's comedies. In Twelfth Night, Shakespeare makes some interesting assumptions about the sexuality of his characters (notably Sebastian, Antonio, Viola/Cesario, and Olivia).
Olivia's previous fascination with Viola/Cesario does not prevent her from marrying Sebastian, however. Some experts maintain that Olivia's bisexual proclivities evidence homoerotic and homosocial behaviors during Shakespeare's time. However, Olivia has to suppress her inclinations because it is against the prescribed social norms of her time. Her conversation with Cesario shows that she is intrigued with the idea of loving an androgynous man but will not admit to it:

VIOLA That you do think you are not what you are.
OLIVIA If I think so, I think the same of you.
VIOLA Then think you right: I am not what I am.
OLIVIA I would you were as I would have you be!

Cesario/Viola accuses Olivia of not knowing her real mind, but Olivia accuses Cesario of the same thing. For her part, Viola loves Orsino, and she appears to betray little romantic affection for Olivia: "After him I love / More than I love these eyes, more than my life, / More, by all mores, than e'er I shall love wife."
So, a marriage between Olivia and Sebastian, while satisfying the gender norms of Shakespeare's time, does little to address her physical attraction to the androgynous Cesario. Pequigney argues that it is in Olivia's "erotic interest to fantasize Cesario as virile." Why? In doing so, Olivia likely feels that she can indulge her forbidden tendencies without fear of recriminations.
Olivia's keen acceptance of Sebastian once she discovers Cesario and Sebastian's shared heritage is telling. 

ANTONIO Sebastian are you?
SEBASTIAN Fear'st thou that, Antonio?
ANTONIO How have you made division of yourself? An apple, cleft in two, is not more twin / Than these two creatures. Which is Sebastian?
OLIVIA Most wonderful!

In accepting Sebastian, Olivia foregoes Cesario. However, she is saved the public embarrassment of loving an androgynous man. Yet, if we are to believe Pequigney's argument that Sebastian harbors latent homosocial and homosexual tendencies (in his relationship with Antonio), Olivia may have an interesting marriage on her hands.