[Acheron]

Live together for at least a year, and have regular sex throughout that time (and, y'know, for a while before moving in together). Meeting the family is also a must unless there's some kind of extenuating circumstance: if one partner has severed ties with their family for whatever reason, there's no point meeting them. Oh, and one anti-milestone: don't marry anyone who's cheated on you.

Not that I'd get married at all, of course. Between the sheer number of marriages that end in divorce and the pro-women divorce laws that give her a claim on as much of my property and future income as she wants, why bother? I don't gain anything by marrying, and I stand to lose a great deal.