Is this a red flag?

I've been dating a woman in her mid 40s for 4.5 yrs now and we've been living together for the last 2.5 yrs. 1 year into the relationship, (we were exclusive and were definitely in love and had talked about a life together) she went out to a bar/restaurant with her girlfriends and ran into an old fuck buddy. She introduced him to her girlfriends briefly and then he went on his way. Later that night, he texted her at 1 am in the morning asking if she wanted to hook up that night. She did say no and that she was practically married and described who I was and said good things about me. They kept texting for another 15-20 minutes and he was asking her if the friends she was with were single. She explained that they weren't. They eventually reminisced about their sexual escapades and she reminded him how good that pussy was. To add further context, this same guy had texted her several months before her and I started dating looking for a hook up and they had a spicy conversation but they didn't hook up because she said she was looking for something long term. I found all of this out after going through her phone one night and then I confronted her the next day. At first, she played dumb acting like she didn't recall this conversation and when I added more detail, it came back to her and she said it was innocent. I gave her more detail and she then apologized for the flirtatious texting and said that she was sorry, and that it was inappropriate. I said to her that I thought accepting his text that late at night was disrespectful to me but she doesn't seem to think so. I feel like the fact that he saw her at the bar and he felt compelled to then text her hours later for a hook up, seems like she left it open. This is a person that I have thought about building a life together and marrying but I am really struggling with it all. Interested in hearing both male and female perspective.