Yeah I would think it would be pretty tricky to decide whose relationships are worthy and whose aren't. Making the bar engagement/marriage assumes that all couples want to/are able to get married. If you go with living together, that would mean a couple who doesn't want to live together before marriage and is having a very long relationship before getting engaged wouldn't be worthy of an invitation, but a couple who jumps into cohabitation after dating 2 months is worthy. Some folks on this thread are saying being together for a year is not that long, but some people get engaged and even married in that short a time period.
I have a relative who in the span of just about one year broke off a relationship of a few years, met a new woman, got engaged and moved in with her, broke up with her and stopped living with her, and started dating someone new. I'd have been pretty ticked off if after 2.5 years with my partner I wasn't invited to bring her to a family wedding because we're not engaged, while his fiance--who'd he'd known a total of about 5 months--got invited. (As it actually happened, I got an invitation with "and guest" on it and he I'm sure got an invitation with "His Name and Fiance Name" on it. So there actually is a sense in which his fiance got invited and my partner didn't.)