Answer:
Yes, it is their faith to only marry people that belong to the Amish church.
- eletias -
It be better to say "yes and no".
Most of time they do marry in their faith, only looking for a mate in their own community, which has increased the progression of genetic problems.
But there is small percentage of outside English, that do join the Amish, Amish adopt English outside children, and rumoured infertile men have outside men sleep with their wives to conceive, plus occasional infidelity.