How do Catholics respond to Matthew 1:25 meaning that Mary did not remain a perpetual virgin?

Roman Catholics believe that the virgin Mary was not only a virgin up until her birth of Christ, but remained a perpetual virgin until her death, but doesn't Matthew 1:25 affirm that Mary and Jospeh did in fact have sexual relations? Matthew 1:25 reads: > And knew her not till she had brought forth her firstborn son: and he > called his name Jesus.