"Relationships that are not eligible
You cannot be sponsored as a spouse, a common-law partner or a conjugal partner if:
* you are under 16 years of age
* you (or your sponsor) were married to someone else at the time of your marriage
* you have lived apart from your sponsor for at least one year and either you (or your sponsor) are the common-law or conjugal partner of another person
* your sponsor immigrated to Canada and, at the time they applied for permanent residence, you were a family member who should have been examined to see if you met immigration requirements, but you were not examined or
* your sponsor previously sponsored another spouse, common-law partner or conjugal partner, and three years have not passed since that person became a permanent resident. "
http://www.cic.gc.ca/english/immigrate/sponsor/spouse-apply-who.asp