Biblically Dense

In all my years of reading the Bible, attending Christian education and studying Church history, one fact always sort of floated around on the periphery of my understanding: Simon Peter (the Disciple) was married. I had never given it a ton of thought, as his wife never makes an appearance in any of the Gospels but I had always wondered where people got the notion that Peter was married.
I was reading 1 Corinthians 9 this morning and I came across verses 3-6, which read thusly:

3This is my defense to those who sit in judgment on me. 4Don’t we have the right to food and drink? 5Don’t we have the right to take a believing wife along with us, as do the other apostles and the Lord’s brothers and Cephas[a]? 6Or is it only I and Barnabas who must work for a living?
1. 1 Corinthians 9:5 That is, Peter

A footnote in my NIV study Bible on Peter being married directed me back to Mark 1:30:

30Simon’s mother-in-law was in bed with a fever, and they told Jesus about her.

The sound of my hand slapping my forehead could be heard from blocks away. How do we know Peter was married? He had a mother-in-law. How many times had I read that passage and simply skipped right over that fact?
I am teh dumb.