In the first stanza, the speaker discusses how he first experienced romantic love in abstract form, as a desire for love. However, a desire for love could not, by itself, satisfy him. The speaker needed an object for his love—in other words, a real person to love. He therefore asks Love to send him a physical woman to love. This is a reversal of how love is usually described. Typically, it is understood that a person sees another person who is desirable, and then love follows, not vice versa.
In the second stanza, the speaker discovers, however, that he was wrong to think he would find love primarily in the physical attributes of the beloved. He discovers that love can reside neither in pure abstraction ("nothing") nor in the purely physical ("things"):
For, nor in nothing, nor in things
Extreme, and scatt'ring bright, can love inhere . . .
In the real world, pure, abstract love ("air") is impossible, and a perfectly pure love (the lover as "angel") is impossible, too, so human love resides in some place between:
As is 'twixt air and angels' purity,
'Twixt women's love, and men's, will ever be.