When their family is complete.
Sparked by two (separate) people I know who recently got pregnant with or gave birth to their fourth child. Both shared freely that it was an accident - contraception failure. Life with 3 DC is full on and a bit of a struggle already in both cases. Neither felt abortion was an option for them.
When you’re 3 DC in and your family is complete, why on Earth are the men not going for the snip to bring an end to it all? And stop birth control being all on the woman?