To own my mistakes in this thread:
-I shouldn't have used 'inadmissible', that was unintentional, I was referring to selection under e.g. express entry.
-Including or not including a spouse can affect the points and hence selection (depending on the spouse's details such as language). BUT what I was under mistaken impression is that if spouse is non-accompanying, it seems it does not affect those calculations or have implications for selection. Mea culpa. (I don't know about other programs well)
However: I believe the application process still requires that one must declare marital status including common law relationships; not disclosing such (if and only if actually common law) would still be a misrepresentation.
But since common law non-accompanying doesn't impact selection (it seems), this may not be something of substance that examining officials would look at in detail...I don't know.
At any rate, at most only one of many factors, and if relationship situation is straightforward (as OP indicates), likely not an issue - in other words, I agree it's not a rare situation and not necessarily a concern for examining officers unless perhaps other aspects of application have red flags or other issues.