A Bond of Annuity in 1805 Haddington

Puzzling afresh over a Bond of Annuity set up by my g…uncle in 1805, providing for his partner and their three children. And also binding his heirs to do likewise. But because they never married his property including a small landed estate was inherited by his younger brother, my ggggg-granddad. Really puzzled why they never married. I’m wondering if the partner was already married. But more to pursue! I also now have a fresh copy of the document, so can see the sums involved. It will be interesting, for example, to do a rough conversion to modern day values. This all took place in Haddington, East Lothian.