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
His new ‘mistress’ is his role as a soldier.  The verb ‘chase’ is an ironic echo of ‘chaste’. His focus and priorities have changed. 
 The  alliterative  ‘f’s in line two, and the long vowels —in 'first’, ‘foe’ and ‘field’ — are emphatic. It is clear his mind is no longer on Locasta, but on the excitement of the future battles.
To Lucasta Going to the Wars