I have seen roses damasked, red and white, But no such roses see I in her cheeks; (lines 5-6)
Here's a moment where Shakespeare pokes fun at an especially "poetic" phrase. Comparing cheeks to roses is already a bit of a cliché, and all of this stuff about "damasked, red and white" just takes it a step further. He uses this lame image to make us see how far from the reality of love some writing can be. Sure we might enjoy these fancy phrases, but what do they really say about our experiences, our daily lives, and our feelings?
I grant I never saw a goddess go; (line 11)
This is also a reference to a whole tradition of fancy-pants poetic language. If you really wanted to seem like a smart, cool guy, you'd toss in a few mythological allusions. Your girlfriend would suddenly become Venus or Helen or Diana. Our speaker will have none of this. He tosses aside the poetic world of goddesses and myths, instead praising his woman who walks on the ground.
…I think my love as rare As any she belied with false compare (line 13)
This is another swipe at love poetry. Those poems do nothing, says our speaker, other than make up false comparisons and pretty lies about women. That kind of flattery doesn't improve anything. It just misrepresents women, who don't need fancy phrases in order to be wonderful or "rare."