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For what it’s worth, my experience leads to this answer:
Only if you have already made the commitment to get married or if both of you do indeed see yourselves married to each other. If you’re doing it for the promise of lots of easy sex or to save money, it is not a great idea.
I have co-habited three times. The first two ended badly. The first time lasted less than a year, but we were young and we did it not only to be together but to save money as we were both moving to a bigger city. Living as a married couple that had no immediate plans to get married just made the situation feel wrong for me. When the arrangement didn’t turn into an everyday sex-fest, I quickly became dissolusioned with the relationship. Remember — we were both in our early 20s, and now in a strange town in a new place making new friends. It didn’t feel like the right time to settle down. On an up-note, she and I are today the best of friends.
The second time we did actually announce our engagement, but I was besotted by this woman — mainly because I didn’t really know her. We moved in together, once again, in a new place for both of us. I ignored the signs that this wasn’t going to work, and despite the huge argument we had a few weeks before, we moved in together, and it went sour pretty fast.
Nevertheless, we stayed together nearly 4 years, partly because of finances and partly because on paper, we should have been perfect together. However, hardly a day went by without some acrimony. Finally, I grew a pair and asked her to leave. I haven’t seen her in 18 years, although someone who is now a good friend dated her after me and will corroborate my experience with her. Crazier than a bag of hammers, he calls her.
The third time was the charm. This time I moved to her place, again in a new location for me. I did it because every day with her brought me peace — something I greatly needed after some personal and professional tumult. I made the decision because I easily saw us married. My wild oats were sowed.
She makes life easier. We never argue, and in fact, until we had our daughter, I never once heard her raise her voice. All my friends, male and female, want their own version of her.
I moved in in September, we got engaged in February, and got married the following September.
In my mind, living with your sexual partner is in no way immoral. It is, after all, a commitment. Maybe it’s not blessed by the church or licensed by the state, but I’m an atheist and I don’t think government should have any role in marriage whatsoever.
I do have a friend that’s been cohabiting with the same woman for 30 years! They have a house, a daughter now in her twenties, and seem to be living a good life. However, if they were ever to break up, they would need a lawyer. To me, if you need a lawyer to break up, you are married.