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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Especially in this day and age where we're all maxed out both mentally and financially, I can see how it would seem to simplify things for couples to just move in together. But now that I've seen so many examples of it being done both ways, I'm convinced that the sacrifices are well worth it when you wait to set up a home until after the wedding.