Don't be too quick to assume that a wedding jealousy fight disaster will kill any relationships. Once sims are BFF they're nearly impossible to split up perpermantly. Some relationships need a little more work, that's all.
Agree with sugoisama (drama for the win! ) but if I didn't happen to want that and I absolutely wanted Don there, I'd make him selectable with mods and keep an eye on him. Maybe even turn free will off if things were getting out of hand.
Nowadays I also have Drop Old Loves, so that when sims get married, their other crush/love flags are removed, so that's another option. Can't remember if it's one-sided, though, and only removes flags on the side of the sim getting married.
"May the sunlight find you, thy days be long, thy winters kind, thy roots be strong." -Grand Oak Tree, DAO