That from the nunneryOf thy chaste breast, and quiet mindTo war and arms I fly. (2-4)
The speaker describes a very pure form of love, one in which sex plays little to no part; the "breast" are "chaste," and the speaker compares them to a "nunnery," a place associated with the surrender of sexual desire (and the potential for boyfriends and husbands) and the acceptance of a life of religious solitude.
True, a new mistress now I chase, The first foe in the field; (5-6)
The speaker equates Lucasta and his enemy in battle; they are both "mistresses" of his. This equivalence suggests that there is something similar about chasing an enemy and loving a woman. This could either suggest that there is something sentimental about war or something violent about love. Either way, it's not the most flattering comparison.
And with a stronger faith embrace A sword, a horse, a shield. (7-8)
We have to wonder just a little at the fact that the speaker embraces the instruments of war with a "stronger faith" than he shows towards his lover. Is this because at heart he loves battle more, or is that war brings out a "stronger faith" that other aspects of life just can't?