When the federal government enacted 12 USC 1701j-3, the purpose, as suggested in the title, was to preempt state legislative enactments and judicial decisions that prohibited enforcement of due-on-sale clauses. As of 1985, all of due-on-sale clauses are enforceable, thanks in part to this statute.
The legislators provided several exceptions to the due-on-sale enforcement, among these is the ability to transfer encumbered property through devise, decent or operation of law after the death of joint tenant or tenant by the entirety under 12 USC 1701j-3(d)(3). Whether intended or not, this creates an interesting problem for couples who have spent considerable time together without marrying. In particular, if one of them owns property that he or she now wishes to leave to his or her long term partner.
Suppose that this property is encumbered by a mortgage and only one of the cohabitants is on the deed. How can that person, the testator, ensure that the property will pass to his or her partner without triggering a due-on-sale clause? A devise under a will is problematic, since only a relative or a spouse is specifically exempted under subsection (d). In states were common law marriage (cohabiting without a marriage license) is not recognized, such as New York and New Jersey, the long term partner is not related and is not a spouse.
The only solution seems to be an inter vivos grant of interest in the property to the partner who is presently not on the deed. Meaning, the person on the deed grants a share in this property to his or her partner as a joint tenant. The problem of course, is making sure that this grant of ownership will not come back to haunt the grantee, since after all, there are no tangible commitments between such partners. Thus, in case G-d word is not reason enough, this is another reason to marry.