How would that provide `more certainty of ability' to remain in Canada? As we all [should] know, a spouse to a Canadian PR or citizen really has no more rights to remain in Canada than a visitor does. Certainly no more than that of a common-law partner.
In practice ircc is less likely to move against an overstaying spouse who has a plan to apply.
And it is different now because the boyfriend is not common law. Yet.
And - say the boyfriend has to leave canada for some reason, and then cannot return (eg entry and or new trv refused).
Them, no way to become spouses (potentially) because marriage not allowed on home country.
No basis to apply for spousal. Scrutiny under conjugal to je expected, because they had opportunity to marry and did not. Etc. There are other scenarios.
Married status provides some - possibly considerable - certainty of ability to sponsor and remain in country.
Has to be weighed against home country risks (in this case) but vis a vis Canada and Canadian immigration, undoubtedly superior.