First date confusion

First time poster here, looking for advice (gulp!). After about a week of messaging someone on Tinder who defines himself on his profile as poly, this person invites me to hang out with him and his "lady friend" at his apartment. Our conversations had been pretty short and informal, so we hadn't discussed the nature of our existing relationships or our intentions toward each other, so I wasn't quite sure what to make of the invitation, though I was feeling spontaneous (and very attracted to this person), so I decided to go. Although I've been in non-monogamous relationships for a long time, I've never met someone from a dating site at the same time as I met their partner or other date, so some of the cues that might have been obvious to someone with a lot of experience dating in various poly communities were not so clear to me. I ended up having a very good time with both of them, but the conversation remained casual, and I thought the vibe was much more one of friendship than of possible romantic involvement, especially as the night progressed and the existing couple got pretty cuddly / physically intimate with each other while we watched a movie together and I sat more or less on my own next to a friend of the couple's who arrived a bit later. The friend and I eventually started to feel like we were intruding on the couple, so we decided to leave, and we walked out together.

To make a long story short, I ended up hooking up with the friend of the person who had invited me over-- a decision likely influenced a bit by the pot I had smoked and the beers I had drunk earlier in the night. I felt alright about the decision at the time, especially since the couple was probably involved in similar activities after I had left the apartment, but the following day, I received a text from the original date clarifying that his intentions toward me were romantic and not merely friendly.

I'm wondering: did I really mess this up? If I'd like to hang out with original date again, how should I bring up the fact that I slept with his friend after our first date? As a sex-positive person, it's been a very long time since I've felt any guilt after a sexual encounter, so I'm struggling with complicated and unfamiliar feelings right now.