Why should a foolish marriage vow

Why should a foolish marriage vow, 
  Which long ago was made,
Oblige us to each other now
  When passion is decay'd?
We loved, and we loved, as long as we could,
  Till our love was loved out in us both:
But our marriage is dead, when the pleasure is fled:
  'Twas pleasure first made it an oath.

If I have pleasures for a friend,
  And farther love in store,
What wrong has he whose joys did end,
  And who could give no more?
'Tis a madness that he should be jealous of me,
Or that I should bar him of another:
For all we can gain is to give our selves pain,
When neither can hinder the other.