Romeo uses two metaphors together:
I'll tell thee, ere thou ask it me again.
I have been feasting with mine enemy,
Where on a sudden one hath wounded me,
That's by me wounded: both our remedies
Within thy help and holy physic lies:
The bolded metaphor compares his experience with Juliet to an eating experience. Eating involves consuming and digesting. Each of these great actions make clear the connection to a love relationship. Romeo demonstrates the internal fulfillment he is getting from the connection to Juliet, although he has yet to reveal which enemy he has been feasting with.
Then, the italicized portion--which I believe is the greater metaphor--compares their love connection to the connection between two enemies. Enemies hurt each other. Using the sense of wounding each other, Romeo asks the friar to remedy the wounding of each other by marrying them. This wounding rings true with love because a cancelled love, or love that cannot be fulfilled physically, hurts. For Romeo the only solution will be a full and complete commitment, just like marriage can provide.