I fucked up this weekend and would love some advice

So this is going to be a possibly long and confusing text post but I will try to keep things as clear as possible. So I am a theatre design student in college and there is this guy, call him Alex, that I recently met on grindr that is also in the department, but he isn't out. We started talking about two weeks ago on grindr, then eventually moving to Snapchat (we decided not to text because his parents aren't accepting and as someone who was outed, I know that's the worst feeling in the world). Anyway, we were talking for a week or so before we decided to hook up. We didn't have sex, but other things. Anyway, the next night was a house party where basically everyone in the department went because it was the last one of the semester, and everyone was pretty drunk including myself. And being the drunk, horny guy I was, I made out with a different guy at the party and he saw it. Later that night I texted the guy I made out with at the party (again being drunk and horny) saying that I had been wanting to make out with him for a long time, which is COMPLETELY not the case. So, then, I try talking to Alex the next day and he basically says I was a shitty person and that I should have ben faithful and I shouldn't have gone and made out with that other guy, even though there was never any indication that Alex and I were anything close to exclusive. So I have a few questions: 1) Was I in the wrong for making out with that other guy, even though we were not anywhere close to exclusive? 2) What are some cute or sweet gestures that I could do to show him that I do care about him and that I'm not a complete asshole? Since I am in the arts, I'm down for something creative, just not something to Pinterest-y or girly because neither of us are feminine acting (but of course there is nothing wrong if you are, that's just not how we are!) Any advice on this would be awesome, if you need clarification I would gladly give it! This is kind of a complicated situation because we work so closely together and I would like to be able to have a good working relationship with him, as well as maybe something more in the future if he can see that I'm not full of shit.