1. It makes it too easy to drift into marriage
Practical problems like financial pressures or roommate issues can make moving in with your boyfriend or girlfriend seem to be the easiest solution, whether or not you're certain that this is the person you want to be with for the rest of your life. Then, as the months turn into years and you're still under the same roof, you naturally start thinking about marriage -- if nothing else because it seems to be the next logical step. If you've been living together long enough and things are going fine, eventually there's a subtle pressure that makes it seem like having a wedding is something you should do. And when you haven't had the space (literally) to take a step back and objectively consider whether this person is truly the best match for you, the situation is ripe for sliding into marriage by default, rather than getting married as an active, conscious choice that you're genuinely thrilled about.
2. It makes the proposal anti-climactic
Ah, the marriage proposal. From time immemorial it's been romanticized as a huge climax in two people's lives -- and most of the romance comes from the idea that the man and woman are entering into a huge new commitment together. A proposal can still be beautiful and touching if you're already living together, but it'll lack a certain gravitas. If you're already engaging in all the intimacy and sacrifice that comes with making a home together, the moment of the big decision has long passed; in a way, your engagement is already over even before rings get involved.
3. It renders most wedding traditions meaningless
Most wedding traditions become obsolete when we view the institution from the lens of secular culture, but a few of our cherished rituals that couples most look forward to when planning a wedding are particularly hollow and superfluous if you're already living together:
The honeymoon can still be a fun getaway for a newly married cohabitating couple, but it lacks the specialness that's there when it's the first time that a couple has spent extended amounts of time together under the same roof.
A father walking his daughter down the aisle has long been a sweet symbolic act of a woman going from her parents' house to the house of her own new family, but even its symbolism becomes strained when she's long been building a home with her new spouse.
Wedding registries were always a way that two people coming from their parents' homes could get a jumpstart on furnishing their new digs; if you're already set up in a fully functioning household, there's no need for those kinds of gifts.
And though I can't say I'd be sad to see this one go, there's no point in hosting bachelor/bachelorette parties when the engaged couple's last nights living on their own happened a long time beforehand.
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4. It sends the message that marriage isn't important to you
I know that most people don't intend to send this message when they move in with their significant others; as I said above, many people I know chose to live together first out of a desire to avoid divorce. However, the message that you and your boyfriend or girlfriend send to one another when you set up house before a wedding is that marriage isn't that important as to be worth waiting for. When you cohabitate, you're implicitly saying that your future marriage isn't valuable enough to be worth tough sacrifices -- and that sets a dangerous precedent for when you do take the next step in your relationship. Combine that with point #1 about drifting toward engagement by default, and it puts a crack in the foundation of your relationship that could take years to fix, if it doesn't spread and get worse over time.
5. It limits your options
Most of the religious couples I know adhered to the idea that they'd never date someone whom they weren't interested in marrying, at least not for long. A friend once mentioned that she had a very nice boyfriend in college whose company she enjoyed, but when it became clear that they weren't meant to be together for life, they mutually and immediately broke it off. When I first encountered that idea it seemed unnecessarily strict, but now it makes a lot of sense. Marriage is the most life-changing commitment you'll ever make, and so it makes sense to order your entire dating life toward that goal. When you're paired up with someone who is not ideal for you, you are missing opportunities to meet the person who is the man or woman of your dreams -- and living together makes it hard to extricate yourself from lukewarm relationships, much more so than if you'd maintained separate residences. Sure enough, just a few days after my friend and her nice college beau parted ways, she met the man who is now her husband of fifteen years, and they have one of the strongest, most joyful marriages I've ever seen.
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