Just found out my partner [22M] messaged someone behind my [22F] back five years ago TL;DR

how would you handle it considering it’s 5 years later? My bf and I have been living together for 3 years and have been paying bills together since 2022. I am just now finding out that my boyfriend of 6 years messaged a random chick on discord in 2020. (We had been together for a little over one year at this point). He was asking to see pictures of her body like shirtless, private parts and she wouldn’t send unless he did. There was zero conversation, zero compliments from him to her, nothing other than him asking for her to send certain pics repeatedly. He sent pictures of himself (that I took) and also sent sxual pic of him grabbing himself with clothes on and sft and short video of himself because that was the only way she was going to send anything. There wasn’t any conversation other than him trying to see her naked. I went back to check timelines too see if maybe we had been broken up or fighting or literally anything and we were perfectly fine. He was actually texting me in that same exact moment. I texted him that day seeing if he was awake and suspicious on why he didn’t text me already if he was up and he said that he was playing his game on his phone so I continued on with a conversation and told him I loved him. (He was actually texting that person as I was texting asking why he hadn’t hit me up since he was awake, he was texting her about 30 mins before I sent him first message of the day asking if he was up) I actually texted him and asked if he wanted to come spend the night (bc that was normal) and that we would come pick him up soon.So he was with me about an hour or so after the messaging with the chick. Mind you we had been together slightly over a year at this point. I haven’t known him as the cheating type. I have never been suspicious about if he would cheat or flirt or anything. Sounds dumb ik but I was just really taken off guard because he has had no history of doing anything like this and it has been 6 years of us together. I don’t think I would have been AS shocked if it said 2019 because times were a little rocky then but to see it happened in July 2020 made me sick. I know it is cheating, not emotional cheating but still trading pics and asking to see certain pics so he can get hrd. The chick blew his phone up (bc he wasnt answering, was going to be with soon at this point and i guess got what he needed done, done.) and he didn’t message back anything until 5 days later and said “Whats up” and he got no response. It’s clear that he didn’t feel bad even 2 days after because he turned around 5 days later. I cant help but wonder what conversation would have been had if she messaged him back. I confronted him about it today. He showed genuine remorse and understood the depth of his actions.All I really wanted was an explanation and he couldn’t give me that. He said be can’t give me an explanation bc he doesn’t remember doing it because it was probably something “insignificant” to him at that time. Guessing he means “I was just trying to get one off it meant nothing” ??? like???? We allow each other to watch porn but the going out of your way to trade pics with a random chick on discord??? Anyways, He seemed like he kept trying to make sense of WHY he did it and mentioned the fact that he was toxic and young and dumb but he wasn’t excusing it or disregarding it and he made that clear. He said he doesn’t want me to view him differently because of this but obviously that’s hard for me. 5 years ago, yes, but nked pics and then seeing me an hour later? Eugh. He told me he wants to keep talking and come to a solution but I didn’t want to bother continuing the conversation after he told me he didn’t have an explanation. The fact that he was remorseful and disgusted with himself helped me feel slightly better but i STILL DONT KNOW WHAT TO DO FROM HERE!!!!!