IRCC cannot force someone to marry in order to sponsor someone. I guess the gray area is about HOW the couple will become common-law partners? Will IRCC challenge the fact that the Canadian didnt move to their partners country to become common-law? Check out the link below and draw your own conclusion as I aint no immigration lawyer lol
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/publications-manuals/operational-bulletins-manuals/permanent-residence/non-economic-classes/family-class-determining-spouse/legality.html
Requirement to be married before submitting the application
IRCC cannot require couples to marry in order to immigrate. However, if they are not married, they must be common-law partners. There is no provision in IRPA for fiancé(e)s or intended common-law partners. The expectation is that a Canadian or permanent resident and a foreign national will get married or live together and establish a common-law relationship before they submit sponsorship and immigration applications.
The issue is that IRCC wants to ensure that only spouses or people in spouse-like relationships are given PR. They don't want to give PR to someone's girl/boy friend when the two people may not be that serious about their relationship - they may date for a few months and then break up, for example. That's fine in general, but you shouldn't get PR on the basis of such a relationship.
If two people are legally married, and the marriage is genuine (i.e not done just to get PR), then it's a clear indication that they are serious about their relationship. Sure, they may get divorced later, even perhaps a few months after immigrating, but at the
time of applying, they are in a serious committed relationship.
If you aren't married, they want you to prove that you are essentially married in all but name. Does the world see you as married - do you share finances, live together, etc. That's why the burden of proof is higher for common-law spouses.
If you want to be married/common-law but it is
impossible to do so because of a legal/immigration bar, they have the option for a conjugal sponsorship. However, they really will ask many questions to make sure you are in fact that serious about the relationship. That's why a recent relationship, or something where you can't really prove that the two of you want to build a life together, will be questioned. The burden of proof is even higher for conjugal relationships, unless there's a very obvious reason why you cannot show such proof (death penalty for same sex couples? you probably don't have many social media posts..)