Is it right for a married man to text coworkers from the opposite sex - during work and after 5?

Let me start off by saying I've been with my husband for a little over 21 years - High school sweethearts.  We have kids and pets - the whole nine.  I believe in my heart that he's a good man. He's a good father and a good husband.  He thinks he's getting old because he hit his forties.  To be honest, he looks very good for his age.  Im still physically attracted to him and I try to boost his ego by saying nice things about him - All true stuff. I love him with all my heart and soul. 

But here's the issue - Last week, he got a text with a Good morning.  Number was local and didn't belong to any of his contacts so I gave him the phone and said here, you got a text. I didn't open it.  He immediately deleted the text and put the phone down.  I asked him who was that? He said he didn't know.  But his whole persona changed.  Like he knew exactly who it was.  I've been with the man for a long time - I know when he's lying so I asked him again. Are you sure you don't know that person. He said no, it's probably someone from work.  

I'm sorry but I felt in my heart he was lying. I knew he knew who that was.  So I went online to see our cellular calls and texts and lo and behold, there were PLENTY of texts from that same number (I have a good memory w/ numbers).  I thought, how can he not know who that person is if he's been testing this person back and forth for 4-5 days.  I asked again.  He then got pissed because I wouldn't take no for an answer.  

Two days later I decided to print out the phone and text list.  I noticed he did not make any calls to this person - just texts.  I saved it and waited to talk to him when he was calm.  Days past by and he makes jokes that I make a big deal out of nothing and that he is not cheating on me.  

I did not accuse him of cheating - I accused him of lying about knowing the number when in fact he did know. The proof was in the print out from our cellular.  

I asked him again and still he say it's most likely someone from work (most of the employees are women!) and it's nothing.  

So today I showed him the list I printed out and he got so pissed! He said I invaded his privacy and that I was treating him like a child. All I want him to do is to be in box so I can tell him what to do. He said he would never forgive me for doing this.  
Mind you, he did say that he knows the person (from work and yes a female) but since he knows how I am (I guess jealous) he didn't want to tell me.  

I feel bad only because I know I shouldn't have invaded his privacy but as his wife, I feel like I did nothing wrong.  We should not have any secrets between us anyway and if he can't do it in front of me, then he shouldn't be doing at all.  We have this rule - we are not to be friends with the opposite sex bc we are both not comfortable with situation.  We've had that rule since the beginning so why change it now? Why does he feel like its right to have female friends to text or to sext (I don't know!) behind my back?

I tried telling him that it can start off innocently with texting good morning, how are you and getting your attention but I'm sure this can lead to something else. 

He hasn't spoken to me all day bc he pissed at me and doesn't want to look at me in the face (what he told me).  I think he's pissed bc he got caught.

Tell me, what would you have done?