We're going to need sex police, sex judges and sex courts. The best part is the certainty involved. Most people don't know when they start being treated as spouses, or to what extent they are so treated. For example, the length of time a couple must live together varies from one year in Nova Scotia to three years in Ontario. Common-law spouses in Quebec don't have mutual support obligations after separation, but those in Ontario do.
Similar disparities exist in property division as well.
No longer would a couple living together find out they are effectively treated as spouses, when that was not the intention. We will return to the days when you actually had to undertake a positive act to gain the benefits and accept the responsibilities of couple-ship. And common-law unions break up more often than traditional marriages. In a way, societal acceptance of gay marriage will have a positive effect for the traditionalists. They will be able to go to their own houses of worship, which will restrict marriage to heterosexuals, without fear of intrusion from the state.