There is no evidence that Paul had Mary and Joseph in mind when he wrote 1 Corinthians 7:3-5. Moreover, Paul permits abstention from marital rights by mutual consent in 7:5.
Only for a short period of time for the purpose of prayer and fasting.
Claiming anyt6hing beyond that is misinterpreting Scripture.
And Paul didn’t write that, the Holy Spirit inspired it and is the actual author.
So why would you think that Mary and Joseph, as a married couple, should be exempt from God’s design for the marriage and family?
Why would it have been wrong for Mary to have sex with Joseph?
But note for a limited time. Context is key with all things.
I’ll ask my question again, slightly rephrased: is belief Mary’s post-partum virginity necessary for one’s salvation, and if so, why?