Actually I think the laws in BC are becoming more progressive with nonmonogamous arrangements. I think there was a case recently where a judge split custody of a child between three people who were romantically involved, cohabitating, and raising the child.
But generally you would only be considered common law in BC if you're in a conjugal relationship.
In practice, if that becomes a point of contention (and I don't think this has been a frequent occurrence), it comes down to how you are publicly perceived. If one person denies the relationship being more than a platonic roommate arrangement, but all acquaintances interviewed claim they were under the impression you were romantically or intimately involved, the court would find that to be a conjugal arrangement.
The law does define conjugal arrangement, and sex is sufficient for it to be considered conjugal, but not required.
