Common law spouses have in Canada and elsewhere acquired many of the rights and duties previously enjoyed only by legally married spouses. However, living common law does still not amount to the same legal existence as being married, be it where family law, tax law or immigration law is concerned.
For immigration to Canada purposes, spouses might both be trying to come to Canada, or one spouse might already be a Canadian citizen or permanent resident, and thus be potentially eligible to sponsor the other spouse as a Canadian immigrant. This post is only about the pros and cons of marriage where one spouse is eligible to sponsor.
Canadian immigration law has now extended similar rights of sponsorship to common law spouses as were previously only available to married spouses. However, being considered common law spouses requires that you have been continuously living together for at least one year prior to your immigration application. This proof of cohabitation can be a hassle. And for the spouse from a country where Canada doesn't like to issue visitor visas, because of what it may perceive as the risk that people of that nationality won't depart Canada after their visitor status expires, common law spousal status may be an impossibility unless the Canadian spouse wants to move abroad in order to co-habitate so that a common law spousal sponsorship application can happen.
Getting legally married means you don't need to prove to the Canadian government that you've lived together for even one day. You might need to show some evidence about the legitimacy of the marriage, but that's a lot easier to do than proving the legitimacy of a common law relationship. Show you had a wedding ceremony, show that your families know about the marriage, show that you care about each other through your communications/gifts/visits/children, and you're probably good to go as far as the Department of Immigration, Refugees and Citizenship Canada is concerned.
But in order to demonstrate common law status, the government usually wants proof of joint bank accounts, life insurance policies, property ownership or leases, and affidavits from yourselves, family, friends and neighbours, in addition to your being able to precisely prove that you've been living together for a year prior to applying. Start co-habitating in Canada, then get called back to your homeland for 3 months because of a sick relative, and you've likely wrecked your chances at common law status. Brief times apart are acceptable, but it's a matter of government discretion to as how much of a separation will be considerable permissible.
You should also be aware that with same sex couples finally having the same rights to marry in Canada as opposite sex couples, the get married or don't get married for immigration debate is now as relevant for them as it is for all other couples.
To be clear, a sham marriage is not going to go over any better with CIC than a sham living together common law relationship. But the bottom line is that marriage will likely make your immigration application process go more smoothly.
If you're committed to never getting married, and can eventually qualify for common law status, the desire for immigration shouldn't make you violate your no marriage principles. But if you're like a lot of my newer couple immigration law clients who have been living together for around a year, are keen to be permanently united in Canada, and are thinking about eventually getting married but are putting off marriage for a couple of years until they are more settled, I always suggest that they carefully consider their position.
If the nature or newness of your relationship means that marriage isn't even on the horizon, then by all means proceed with a common law spousal sponsorship application. But if you have been talking about marriage anyway over the last year, and are thinking it is something you want to take care of very soon after the immigration process is finalized, you should think about whether it might be possible to get married prior to applying for immigration.
Marriage won't cure a shaky immigration application, and common law spousal status won't wreck a solid application, but marriage may make things easier. Just some food for your collective wedding cake sweet tooth.