Till Death Do Us Part [Tomarry]

Tom and Harry are soulmates in every sense of the word. They are partners in crime (surprisingly, it isn't usually murder), lovers, and there was that whole fiasco with Harry being a horcrux at one point, so.

They would both say "Till death do us part" but the fact that Harry is the Master of Death, and Tom is his soulmate, means that neither of them can actually die.

It's much harder to get bored living forever when you have somebody to share every moment with, after all.