How do Catholic Proponents of Priestly Celibacy Interpret Peter having a wife?

Catholics would say that Peter (by virtue of being first Pope) is a Bishop. How would such Catholics (though not all Catholics hold to this view) that say Priests must be unmarried and celibate respond to Matthew 8:14, which says that: >14 And when Jesus entered Peter's house, he saw his mother-in-law lying sick with a fever. (ESV) Note the use of "mother in-law". This implies that Peter was married. Again, how do they respond to this?