[color=navy:k7h7tqfv]Similar vows are done today. Couples (usually older couples who have already had kids) do in fact take vows to abstain for a purpose. So no, it’s not a sin to abstain.
As for Mary, well you can basically see it as a nun who married for the purpose of not being stoned to death. Her vow was taken long before she married and it’s completely reasonable to think the older aged Joseph was aware of it. [/color:k7h7tqfv]