AITA for being with someone my friend kind of liked??

So I knew that my friend (We’ll call her Shelby) had a very strong liking for this one guy who was my best friends cousin(We’ll call him Mark). Well my best friend at the time(Well call her Jessica) thought that i should go over to her house. I was excited so i agreed and we went. Jessica told me Mark would be there but that he’s probably going to be asleep the whole time so he wouldn’t bother us. I get to her house and after letting Jessicas dogs i stand up and Mark is standing there. It was a little awkward. Jessica decided that we would take Mark with us now because Jessica’s mom told us too. So we went to go eat food and there where laughs shared and obvious chemistry floating through the air. Mark and I had obvious chemistry. I was excited as Mark would be my first realationship. A week later I see Mark again and he asks me for my phone number. I say yes because i liked him. We texted and our realationship grew. After a few days of texting him i realized something…. My friend Shelby had really really liked Mark. But she told me that she didn’t like him anymore weeks before i even thought about getting together with Mark. Shelby is a more sensative person and so i decided to call her to tell her to try my best to be respectful. She seemed a little frustrated but not nearly as bad as i thought. I was confused by this frustration though because i thought she didn’t like him? Well I get off the phone with her and about 20 mins later i get a call from Jessica. Jessica told me she just had Shelby calling her cussing at her because of how mad she was at me for being with Mark. Long story short she was very mad. Fast forward about a year and Mark and I are no longer together and ended on very weird circumstance but that’s besides the point. Shelby tried to comfort me in the during the break up and she would talk about ”her trauma“ with him. Shelby told me that her and Mark facetimed many times and texted and was practically in a realationship off and on for two years. I confirmed with Mark and found out that that had never happened. I don’t understand why she is lying to me about their realationship that never happened. (Keep in mind Mark had rejected Shelby a couple times in the past) So am i the a hole for dating someone who my friend said she didn’t like but decided she did the second she found out i was dating him? Comments pleaseeee

I apologize if this is not very organized lol this is my first time posting 😅😭