My father had a daughter loved a man / As it might be, perhaps, were I a woman / I should your lordship
Viola hints to Orsino about her true feelings for him. Orsino does not pick up on the clues because he believes Viola is a man, and that the two of them are speaking man to man.
Orsino assumes Viola is talking about a sister but the audience knows Viola is really speaking about herself.
However, even by disguising her comment with reference to a woman, The quote has potential homoerotic connotations in that a man seems to be hinting at his love for another man.