TELL me not, Sweet, I am unkind,
That from the nunnery
Of thy chaste breast and quiet mind
To war and arms I fly.
True, a new mistress now I chase,
The first foe in the field;
And with a stronger faith embrace
A sword, a horse, a shield.
Yet this inconstancy is such
As thou too shalt adore;
I could not love thee, Dear, so much,
Loved I not Honour more.
A very powerful and most quoted part of the poem. He says that he could not love her as much without loving honour more. 
 Perhaps his honour gives him the depth character to love her, or causes him to love certain attributes in her. 
 The affectionate salutation ‘Dear’ is capitalised, as it is in the opening stanza when he refers to her as ‘Sweet’. He may do this to demonstrate her importance to him.  It matches the capitalised ‘Honour’ His love of both are intertwined and inseparable.
To Lucasta Going to the Wars