my fiancé and i just had our first threeway

ok so im engaged to this amazing lady named jessica and we r pretty traditional. she wants kids, marriage and the whole thing. but last week while shes away visiting family, a friend of mine from med school invited me over for dinner and drinks. its been years since i saw her but when she opens the door and we hug, my heart skips a beat. something about her has always made my blood boil even back in college. anyways after some small talk she suggests we play truth or dare and i think its just innocent stuff til she asks me if im still married to be faithful. "are u kidding" i laugh but then i notice how much more open her body language is, like she actually wants a real answer. i tell the truth saying yes but also that i once cheated on an ex with a guy and havent since. the game turns into something else pretty quick as drinks are opened and my friend starts whispering naughty things in my ear, talking about how much shes fantasized about us being together, even mentioning jessica in our fantasies. the wine flows and i know im getting tipsy fast but suddenly she's pushing me onto a bed and its just instinct to flip her over and start kissing her neck. the next day is brutal. shame, guilt, questions of who im supposed to be as a partner and husband-to-be are swirling in my head. jessica was mad but also kinda turned on when i told her the truth last night. she said shed always wanted to try an open relationship. so now we have this big decision: do i go back to our old ways or explore more with both of them? any thoughts from u guys?