My Mistress' Eyes

I am struggling with this piece of poetry. I understand that the beginning is supposed to be satirical. It mocks those men who lie to girls, telling them they are perfect in everyway. We all know this cannot be possible. Not everyone's "eyes are like the sun", and not everyone's skin is "white as snow". The speaker is trying to be realistic in describing his mistress. The part that I do not understand is why?! Why would the speaker call his mistress these things to her face? I know if it were me, LIE TO ME! No girl wants the truth! To all the males out there, LIE TO HER! We aren't perfect by any means, but we like to think so.
The last line of the poem does make a small compliment. "..I think my love as rare/As any she belied with false compare." The speaker is insisting that he loves her just as much, be he is not going to lie to her.