I gather that prior to a vasectomy, the patient can select whether the surgery will be permanent, or of a kind easier to reverse. Nothing too technical about it, just surgical technique.
This being said, monogamy is no solution, as even if he honestly did not have sex with the woman, she can extort him by claiming he had. Typically this means he will be under pressure until the baby is born and comparative DNA tests can be made. Which is a very long time to have a ruined reputation, severe family friction and even divorce, with little or no chance of monetary recovery from the blackmailer.
Nonsense. They can do paternity DNA almost right away.
And vasectomies are reversible, to my knowledge.