<human>: Give me a shortened version of the text

I've been with my girlfriend a little under two years now. The relationship has been great, aside from the occasional argument, until recently. 

A few days ago, I was hanging out with my girlfriend when she made an off-hand comment about something bad she'd done. She clearly felt guilty and didn't want to tell me, but I didn't let it go and eventually it came out that she'd kissed her friends at a party she went to a few weeks ago. This is essentially what she told me:

It was at a small party with a few of her close friends. They'd been drinking, and eventually started playing spin the bottle. She joined in (she told me that she didn't really want to, but everyone was playing so she went with it). She made out with a few of them, and nothing else happened.

I told her that I felt that even kissing is like cheating to me, but she didn't think it was a big deal. I mentioned that she shouldn't have joined in even though everyone else had, but she brushed that off. She also said that it was just her friends and "it's not like it was another man" but she's bi so the gender doesn't really matter, right? I eventually just gave up and ended the argument, but it still doesn't feel resolved.

After talking it through she did apologise and tried to make it OK but I still don't think that she really agrees that what she did was wrong.

I still don't know what to think about this. We talked it through but I still feel betrayed. So my question is basically: Is kissing like a 'lower' form of cheating? I'm just not sure if I'm making the right decision by forgiving her and ending the argument.
<bot>: My girlfriend made out with her friends, not sure if it's technically cheating and if I should feel betrayed or just let it go.