How to deal?

Okay, first let me start off by saying that if you are one of those women who never feels threatened or insecure in her marriage by anyone or anything, good for you, truly. I do admire that quality, however all women are different and I am simply not that type of person. Like many, I tend to get jealous or can be made to feel insecure under what I would consider appropriate conditions. 
Anyway, this morning my husband was in the shower and left his phone on the bed. It is very rare, but occasionally I do look at his texts. He is a wonderful man, however we have gone through our rough patches prior to getting engaged and married, and I think that has scarred me a bit in terms of trusting him 100%. It happened a long time ago, but every now and again I do feel a need to check in and make sure nothing suspicious is going on. 
So, for some reason today I felt the need to do a quick scroll through. I know a lot of people are going to say shame on me for snooping, blah blah blah. However, I believe that in a marriage you have every right. I also feel that he has every right to look at my phone, should he want to. So, moving on from that..
My husband is a manager at a car dealership. As I was scrolling through I see a message to his co-manager about how it's "The Alex show today and she sold three cars so far." Instantly I was a little taken aback because I was not aware about any female salespeople... so it peaked my interest. I really wasn't too concerned but I felt like there was some type of tone there in how he was talking about this mysterious Alex girl. So, I went back and scrolled further and found a conversation between him and her.
All of the texts were pretty benign and the conversations work related, however I noticed that every text he sent her had a smiley face in it. There was one brief conversation between them with him asking her if she had left for the evening, she said yes, then he told her that another salesperson would take care of her clients that had come back in from the previous night to purchase a car. She asked him if he could let her know if it sold, and his response was "No :)" I'm sorry, but that's flirting! 
Then I dug further and decided to do some Facebook stalking. I'm not a hateful woman, so I'm going to be truthful and say that she's adorable. Maybe 23, awesome body and a cute face. Then I stopped myself for a second and tried to flip it around. I asked myself how I would feel if this same conversation was between him and a male salesperson. I'm telling you, if he was sending those texts to a man with all those smiley faces, I would be questioning his sexuality. So, that's my grounds for labeling this situation as flirtatious. The first text on there is from June 13, so it's been almost a month of this (that I can see).
Oh, and I'm also due with our first baby next month. So, this situation has been a little tough on me to say the least. He knows I'm upset but he has no idea what it's about. We haven't talked at all today and I know it will be discussed when he gets home from work. I don't want to deal with this out of anger and spite because I feel like it makes me less attractive to him and her probably more attractive and appealing. In all honesty, I want him to feel guilty more than anything else and to realize that the way he communicates with this girl is wrong in the fact that it is disrespectful to his marriage and hurtful to his wife. I know that if she situation was flipped, and I worked with some ripped, hot young 20-something stud that I was texting with a bunch of smiley faces at the end of every sentence, he would not be pleased.
I'd love to hear some input on how to deal with this maturely while still making him stop and essentially put his tongue back in his head. I understand there will always be beautiful women in the world for him to look at and notice, but as a married man he should not be communicating with this girl the way that he has been and I'd like to be able to make him stop and think about that.
EDIT: I'm not here to argue what is and isn't considered flirting, thanks. In my world and my marriage, my husband is flirting with this girl. Whether you would feel the same way in this situation is not relevant to the post. I'm asking for advice on the best way to handle the situation. 👍🏻