Sex with one's husband isn't impure. There is nothing inconsistent with having children and remaining pure.
The commandment to "be fruitful" didn't have an asterisk (at least not in my KJV Bible). It's possible to be fruitful and pure simultaneously.
So glad you brought that up. I have asked that question a number of times and never received an answer; Why does Mary have to be a perpetual virgin?? Why would having other children make her any less in their eyes?
No...sex with a spouse is NOT impure, but remember the words of St. Paul where he exhorts people(clergy) to be unmarried like him if possible, but if not then it better to "marry than to burn"