Am I being unreasonable if I ask this of my boyfriend?

My boyfriend and I met on a dating app and have been together a little over 3 years, living together a little over 2 years now. He still texts a woman he met on the dating app before me. They went on a couple of dates and had sex, but ultimately decided the 2 hour driving distance between them was too much. However, when he texts her, it is often flirting or even what would be considered sexting. Last September he went on a sudden trip by himself to drive. He had told me that he liked to drive long distances when stressed, and it had been a particularly stressful time, and he had just gotten his car back from the mechanic after about 6 months of not being able to drive it. However, I've recently confirmed that the town he stayed in while in Michigan is the town she lives in. And I saw some of their texts from around that time, and it certainly seemed like the sexting was talking about what they wanted to do to each other, and she seemed to be aware he was driving to Michigan. However, when I confronted him about it, he said that he did not see her while there. He just went to that specific town because he had stayed in a hotel there before. At the time when I confronted him about this, I wanted to go through the texts with him and have him explain certain things to me, because what I'd seen seemed like planning, but he said it was just fantasy. However, by the time I got to where he was, he had deleted the text chain and said he deleted it and her contact information to prove that he was with the woman he wanted to be with (me). He also apologized for the sexting because he said he hadn't thought through how that would make me feel. Today, I saw that he is still texting her. A quick glance at the text chain didn't show any sexting, but it was still a little disconcerting and made it seem like the deletion of the text chain before was just to hide those earlier texts. My brain keeps getting stuck on certain things about those earlier texts too. Two weekends before he went was when he was first planning to do the long drive, and that weekend she texted him "Are you on your way?" and he responded, "No, I have to work tonight. Sorry." Why would she even know he was going on there, and why would he apologize about not going if they weren't going to meet up? Then the weekend that he did go, she texted, "Are you almost here?" and he texted, "Running late, had to gas up." Again, why would she need to know if he was almost there if he wasn't going there to see her? So, after quite a while of my brain circling the drain about these texts, I think I've hit upon a way to get an answer about whether or not he went there last September that doesn't require me just trusting him when he says he didn't. My thought is to ask him to text her like usual, and then to say sometime like, "How long has it been since we've seen each other in person?" Then when she answers, he can just say something like, "Time really flies" or "So much has happened since then" or something like that, and none of that would be far out of the realm of their normal texting. But if he doesn't direct her to an answer, then her answer would likely be honest, since she will be answering him, and I'll get to see what that answer is. Is it unreasonable for me to ask him to do that? This topic has come up about 3 times since I made the discovery of the "sexts" on his phone a few weeks ago. He's been very calm when discussing it, but today we talked via text and I pushed a little harder than I have in the past, and he got angry and stopped responding to me. So, I'm not really sure how he'll take this.