There's a Way Around All This You Know...SPOILERS!!!
The answer is simply never marry. You can screw around, get pregnant or get a girl pregnant, have kids & not marry anyone. I'm not sure the particulars of the deal with the devil was that you HAD to marry. It was never explained if "new additions" meant husbands/wives or even your betrothed.
If you had to try & successfully kill each new bride or groom, then how would the family ever grow & not be doomed to eventually die out? Unless Satan promised 100% fertility rate on the honeymoon. But on the other hand children are considered "new additions" to the family so did they have to hunt those as well?