How is that so you might ask? Well, your own church likely forbids at least two of the following categories of people to marry: those too young to consent, those mentally incompetent and too feeble-minded to consent, those who are not free to consent because they are already married (prohibitions against bigamy, and against divorce and remarriage), and those who are not free to marry because they have taken a voluntary vow of life-long celibacy.
You forgot degrees of consanguinity (i.e., brothers and sisters, first cousins, etc.).