I've been traveling across Latin America and recently spent a month in a specific city in Argentina. I began to see a girl (Girl #1) and we had some chemistry. I knew she was interested in taking things further, but was super busy with school and work and couldn't hang out as often as I would've liked (I had lots of free time).

So, I met someone else on Tinder (Girl #2). When I met this girl in person, I wasn't super attracted to her, so I felt we could just be friends. I didn't mention this to Girl #1, for two reasons. One, I wasn't particularly interested in Girl #2 sexually, and two, I didn't want to blow things with Girl #1 by telling her I was seeing other people.

Long story short, Girl #1 and I hook up a few times, and later Girl #2 makes a move and I say "fuck it" -- and do just that. Near the end of my time in the city, Girl #1 asks if I slept with anyone else while I was there. I didn't want to lie, so I said yes.

As you can imagine, she was upset. She felt that by fucking Girl #2 I invalidated everything that #1 and I had (#1 had divulged some vulnerable things, such as the fact that I was the first person she has felt comfortable sexually with in a long, long time).

Now, from the beginning both of these girls knew I was leaving, implying that I wasn't looking for anything serious. I was up-front with Girl #2 about seeing someone else. She's into polyamory and had no problem with it, but again, I also wasn't even sure I wanted to sleep with her.

The problem is I wasn't upfront with Girl #1 about "seeing" someone else, and only came out about it when it was brought up. This really hurt #1, something I'm not proud of and would like to avoid in the future.

I'd like to hear your thoughts on the situation. Should I have specifically told Girl #1, before anything sexual happened, that I wasn't looking for anything 'serious' or 'monogamous'? Should I have said that, while I'm in the city there's the potential that I may sleep with someone else?

My worry is that by laying this all out early on, I'd be shooting myself in the foot and potentially putting her off wanting to progress the relationship. However, maybe the moral thing to do would've been to face the consequences of whatever path she chose if she'd known this information.

Any general tips for sensitively dating multiple partners (short or long-term) and not having shit blow up in your face?

Thanks!