I'm writing about a world where there is magic and one of the basic uses is as birth control, it is relatively cheap to obtain. I want to know the effect birth control would have on the concept of marriage during the time period
As the comments have pointed out, birth control wasn't unknown nor unacceptable to medieval European peasants, though their version of abortion was to induce a miscarriage. That said, peasants didn't really use birth control because children were a resource for a peasant family - free labor to work the fields, for instance. The expression appropriate to the time period is 'Children are a poor man's riches'.
Among the nobility, birth control would have been more useful (don't want to get caught having an affair with an unmarried woman, for instance), but since they had access to a rudimentary form of it at the time, it probably wouldn't have that much of an impact.