Twelfth Night Gender Roles

According to my classmates, if Olivia finds out Cesario is actually Viola, she won't be able to love him anymore because two girls can't love each other. I think that's absolutely ridiculous. 

The play ends with "Cesario" revealing that he, or well she, is actually Viola, and she gets together with Orsino, who had been pursuing Olivia for quite a while. Olivia gets together with Viola's brother, Sebastian, which I find dumb, because Olivia had started to fall for "Cesario", and as soon as the gender was revealed, she stopped loving her?

Love goes beyond gender and the Twelfth Night would've been better if it were lesbian instead.