Two straight guys with fiancées wanted to make out with me. Why?

So I was in a club and was approached by a guy who immediately assumed I was gay which is true. He complemented me and said I was really beautiful and then we made out. I asked him if he was guy but turns out he's actually straight and has a fiancée but found me very attractive anyway. Then I met his buddy who is also straight and also has a girlfriend and he also kept going on about how beautiful he thinks I am and we made out too. I mean I don't mind making out with straight guys, I found it very flattering that two heterosexuals would kiss me despite their sexuality and society's expectations on conventionally masculine behaviour. I'm just wondering why they'd wanna do that if they're straight and on top of that are in really serious relationships with women. It's kinda odd for straight men to appreciate another man's looks let alone get physical with one. What do you think why they did that? And was it immoral for me to make out with them despite knowing they have fiancées? I don't really like monogamy and technically I have no responsibility to help anyone preserve their exclusivity but maybe that's a selfish way to think.

 

Well yeah, but how is it my responsibility when I didn't promise anything to anyone? I personally would never pursue a monogamous relationship anyway as I don't believe in monogamy and I also don't want to support that.