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.
Inconstancy means unfaithfulness or inattention. But this kind of unfaithfulness is considered something she too will appreciate along with him.
To Lucasta Going to the Wars