when i crossed the line

so i guess this is me trying to figure out where exactly it went wrong. im in my 30s and been with lily for about six years now, open relationship kind of thing but we dont really do much swapping or anything - just a sense that its okay if something else comes up. anyway, the problem started last month when i was at a party and met tina. she's like this super confident girl in her mid 20s who i kinda hit it off with over drinks one night. we went back to my place for a while, just talking and stuff, but it felt different than other times ive been out with someone else. lily knows about tina and i told her straight up what happened but also that nothing was going to come of it, you know? or so i thought. a couple weeks later i saw tina again at a bar on my own and we got talking - still no plans or anything but it felt like there was this tension building between us. one night she suggested we meet up and explore what could happen if we wanted to, just once. lily knew all about it so its not like i was hiding anything, right? only thing is, as soon as tina came over it wasnt even about exploring anymore - it was this intense heat that took us both by surprise. next thing i know were in her bed and the line gets crossed before i even know what im doing. afterwards we were both kinda shaken up but decided to just go with it for a night since it happened. no big deal, right? wrong - lily found out about it later that week and now she wants us all to talk it through as a threesome or something but the thought of seeing tina like that again makes me feel sick inside even though i know its my fault for getting caught up in her first time.