Why didn't Northern states think slaves should be counted
for representation purposes?

The Southern states wanted to count the entire slave population. That would increase their number of members of Congress. The Northern delegates and others opposed to slavery wanted to count only free people, including free blacks in the North and South.

Explanation:

in griswold v. connecticut (1965), the supreme court ruled that a state's ban on the use of contraceptives violated the right to marital privacy.