My face in thine eye, thine in mine appears (15)
In line 14 the speaker tells us that he both is a world and has a world (his lover). The same goes for her. But in line 15 we get a high-definition description of just how close this couple is. Not only are they a pair of different worlds, ready to be erotically explored; they are also mirror-images of each other. When he looks into her eyes, he sees a reflection of himself.
Whatever dies, was not mixed equally (19)
Bad unions are out of whack and unhealthy. In fact, when anything dies it's a sign that it contained disorder. Feeling under-the-weather this morning? Looks like your bodily humors are out of balance and if you don't get them back in the game, you'll probably die.
If our two loves be one, or, thou and ILove so alike, that none do slacken, none can die (20-21)
It doesn't get more perfect than this, folks. This couple's love is so balanced and shared that it's like good news at the doctor's office: healthy and disease-free. Plus, this union is going to be bright-eyed and ruddy-cheeked for a long time. It's never going to weaken and it's never going to die.