The ironic twist is the play of what is to be expected to be said and what is actually said (or, going back to the argument, what is expected from love and what actually occurs): It begins: "My mistress' eyes are nothing like the sun; / Coral is far more red than her lips' red" From here the sonnet continues with a much less pleasing list of the qualities about this mistress, who is definitely very far from the ideal perfection noted in the Petrarchan sonnets. The distinction between the two sonnet approaches increases in the last of the couplets when Shakespeare makes his final argument and explains why he has been using such lesser quality comparisons all along: "And yet, by heaven, I think my love as rare/as any she, belied with false compare."

In other words,...

The "real thing" is expressed this way: "It is what's inside that counts," or "Beauty is only skin deep."
Thus, Shakespeare is not demeaning the real beauty -- the woman without the coral lips or rosy cheeks or musical sound -- but rather he is diminishing the woman who is given all these attributes. In brief, he is saying that true beauty, true love, does not have to be exaggerated and made into something that it is not. By itself it is enough. Only something that is false and illusionary and soon to disappear needs to be given structure and meaning.

In tribute to the song group, the Beatles', 50th Anniversary, there is a wonderful line in a song by Paul McCartney that sums up this Sonnet. "Will you still need me, will you still feed me, When I'm sixty-four."