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
The speaker, we can assume also the poet, doesn’t want the woman to think he’s unkind because, as a soldier, he’s must leave her to fight. . 
 The ‘nunnery’ of her breast implies chastity — an idea drawn from medieval romance poetry, where the woman was always a pure, distant, adored person, whose personality was never truly known. Here, Locasta’s deeper feelings or response are also not revealed. 
 ‘Quiet mind’ can mean ‘peace-loving’ or a calm temperament; an ironic contrast to the speaker’s compulsion to fight. 
 The reference to her ‘chaste breast’ has been seen as ambiguous, suggesting that the poet, for all his respect and gentleness, has erotic feelings. 
 In some versions of this poem ‘Sweet’ is bracketed, suggesting that he has to interrupt his thoughts to emphasise his love.
To Lucasta Going to the Wars