How exactly would you go about enforcing that? A few points to consider:

• STD tests are quite invasive. They typical involve two blood tests, oral swabs, anal swabs and a urine test, and even this might not cover everything. There isn't a simple or non-invasive tests that exists that can test for everything. For this policy to be effective, the government would need to mandate that Canadians get tested at regular intervals, which I suspect might not even be constitutionally permittable. The alternative would be to mandate that if you are aware of your STD status, you need to disclose it. But the likely effect of that policy would be to discourage people from getting tested.

• Certain diseases go "dormant" for periods of times, and can be very difficult to detect during these periods. So false negatives would be an issue.

• Generally speaking, it is not recommended to test for certain STDs, because the diseases are very prevalent in the population, and they're generally of no medical consequence. For example, HPV infects about 75% of Canada's population, and most strains are medically inconsequential. Herpes infects about 60 to 90 percent of the population, in 90%+ of cases has no notable symptoms, and transmission is difficult to prevent, since that disease can be transmitted non-sexually, through kissing or otherwise swapping saliva. Standard STD tests will NOT test for herpes because that disease is so common.