Despite all the romance, love in this poem is a very chaste and non-sexual thing for the most part. There are references to a "nunnery" (2) and the word "Lucasta" itself comes from two Latin words and means "chaste light" (lux and castus/casta). In other words, the speaker is at pains in this poem to paint a picture of pure almost heavenly or celestial, love… that he's willing to ditch for battle.
Title: The name "Lucasta" comes from the Latin words for light (lux) and chaste/pure (castus/casta). Awesome or awkward nickname? You decide.
Line 2: The speaker calls Lucasta's "breast" a nunnery. While most ladies probably wouldn't take that as a compliment, here it doesn't sound so bad. And obviously, they're not literally a nunnery so he's using a metaphor to describe the chastity and non-sexual character of Lucasta's body.
Line 3: The speaker calls Lucasta's breast "chaste." That's a new one on Shmoop.
Lines 11-12: The speaker says he couldn't love Lucasta if he didn't love honor more; this suggests something pure about his love for her. It's tied up with his love of honor, and that's definitely a good thing.