Beshrew that heart that makes my heart to groane
For that deepe wound it gives my friend and me;
Is't not ynough to torture me alone,
But slave to slavery my sweet'st friend must be,
Me from my selfe thy cruell eye hath taken,
And my next selfe thou harder hast ingrossed,
Of him, my selfe, and thee I am forsaken,
A torment thrice three-fold thus to be crossed;
Prison my heart in thy steele bosomes warde,
But then my friends heart let my poore heart bale,
Who ere keepes me, let my heart be his garde,
Thou canst not then use rigor in my Jaile.
And yet thou wil; for I being pent in thee,
Perforce am thine and all that is in me.
The first quatrain paints a picture of not only the poet but also his friend in love with the same intractable woman. Groane, wound, torture, slave, slavery all in the first four lines.
The second quatrain contains an extended mathematical play on how this triangular situation can be construed, how he is divided from his own selfe, and from his next selfe (his friend), and how this creates thrice threefold torment for him.
In the third quatrain, the idea of a prison is introduced, and the poet plays with the idea that possibly he could bail his friend with his own heart, in which case his heart becomes a 'guard' and consequently the woman would not use 'rigor'. And yet...
The final couplet admits that because he is trapped (pent in), he therefore belongs to the woman (to do with as she pleases).
It sounds like sado-masochism to me.