More often than not, the actions of others - even those closest to us - are not directed at us personally, but taken as the consequence of something that is driving them (in many cases, the consequence of behavior and attitudes learned and absorbed in childhood), such as an inability to be genuinely intimate.

I’ve considered this question. If Ms gf is cheating, I’m not just going to consider that one fact. If it looks as if we’re unhappy together and we are better off parting ways, that’s fine. We should split. If she got drunk and cheated in a totally out of character way, when she sobers up I would like to talk about her drinking.

Back in the day, when my imagination was a lot more fertile than experience counsels, at times I was attracted by women other than the one I was with. I’m familiar with the “it’s always greener on the other side” syndrome. This delusion probably comes from the insecurities that stand in the way of developing true nurturing and fully supportive relationships with our significant others. To the extent that we judge and compare them to others or to the extent that they judge us and compare us to others we let fear and apprehension get between us. This is no good.

When we end established relationships that have a lot of history it’s never because of one fact. If we make ourselves believe that just one fact requires us to split, we’re deluding ourselves.

Same thing I did with my wife...get rid of her! (legally, I mean)

The two times my lover dumped me (temporarily) to check out another man, I figured, well, it's her life, she's doing what she needs to do, she's doing what I would do in her place, she's not doing (and has never done) anything she didn't tell me she'd do (she had told me many times that she was making no promises - yet), and the last thing I want is a woman who's sticking with me because she thinks she has to, or feels some sense of obligation, or because of money, or is wondering to herself, "...what would have happened if...? ...maybe I should have checked out that Bob guy? etc. Each time, it hurt, but a man has to be able to take rejection if he wants to call himself a man, so I picked myself up, dusted myself off, and got on with my life -- dated some nice women for whom I almost left her, got over it -- so when she came back to me, I think neither of us had a sense of needy dependence -- we're together because we want to be, not because he have to. BTW she has told me I am "a jewel" for allowing her the freedom to do that. Perhaps some women like possessive, controlling men, but this one doesn't.
This is important: any kind of secrecy or deception is highly corrosive, so it helped that the first time, she told me in advance, and the 2nd time she told me very promptly. She's been on the other side of deception, and I know how it hurt her. Also, she forgave me when I had, let's say, some unexpected difficulty extricating myself from a previous relationship (and I'd not been nearly so prompt in telling her). And when we first met, she was very upfront telling me that I was not the only guy on her shopping list. This openness really, really helped. We'd each had the experience of volunteering confidences that we thought had a good chance of torpedoing the relationship, and when those were accepted/forgiven, it built a lot of trust and security.
That fact that we're both over 60 matters (we met in our 50s), so we've been around the block a few times and understand more than young lovers might, and our futures are different. We've been through loss and breakups and betrayals and death, so we know that loss is painful but not the end of the world. It must be harder for young people to understand that.
I think it's tragic that young men fall into the trap of possessiveness and controlling and male supremacy. That just poisons intimacy. I met a guy on the trail once, sharing the kinds of confidences you sometimes do with strangers you'll never see again, and he told he how his girlfriend confided that she's "cheated" on him (I dislike that term), and he almost immediately dumped her -- walked out to the nearest town, and terminated the relationship and the backpacking trip right then and there. Thinking on it, I almost dumped my pack and ran back to catch up and ask him why he didn't simply forgive her, and let that be a trust-builder instead of a deal-breaker? I mean, punishing someone for voluntarily telling the truth does not encourage honesty. Neither does withholding the truth.

So I think the best response to the discovery of unexpected infidelity is to sleep on it, calm down, you're entitled to your own pain but don't let it cloud your judgment, and then talk about the elephants in the living room. Forgiveness is crucial, absolutely vital, because hanging on to resentment is like drinking poison to make someone else sick, and if you're unforgiving, you won't get honesty and that will eventually ruin everything. You may find that (s)he was:
Indulging in rather innocent curiosity;
Finding out that (s)he doesn't really love you that much (in which case, losing her is the best thing that can happen to you);
Finding out that the grass is not greener on the other side of the fence, and you're the best thing that's ever going to happen to her/him;
Succumbing to momentary temptation;
Is perhaps incapable of fidelity ("a hard dog to keep on the porch" ).
... or something else.

To anyone on either side of this, I'd advise honesty and full disclosure, because guilt and secrets are toxic, and especially painful when discovered unexpectedly. I'm not 100% sure I'm right about this -- there MIGHT be times when tactfully overlooking something be be the best way -- but if it is safe to do so (for women, it sometimes isn't), confronting the truth head-on is usually best.
There are better and worser ways to get through this. Choose the better ways.

You gotta wonder, what else is she lying about? Put the shoes to her. Don't look back. Burn the bridge. Five gets you ten she cheats on that guy too.

Ask her about it and see if we can work through it. I'm not possessive or jealous.

Ditch Her!

I asked her to leave

Let it pass. Sex is not the be all and end all in a relationship. ?

It would depend on the seriousness of the relationship. If somehow she convinced me to have a strictly exclusive arrangement, I'd feel betrayed. And just become increasingly jaded and resist that form of relationship in the future.

I think it’s specific to your relationship and circumstances; however, if you had an established notion of monoamory, then she’s absolutely breaking your trust and you should tell her as much. Even though she presumably did break your trust, you were with her for a reason and I believe should internally wager that against your feelings of betrayal. Whichever one is stronger should lead you toward what to do.

I'd leave the relationship.
Not sure what kind of answer you're looking for, but I'm not a violent person or a vindictive one. I wouldn't waste any more time on someone that didn't respect me and I would try and not waste my time being angry.

Question my sense of ownership.

Either this question is too awkwardly worded or I just really don't see the point in it? Either way I don't quite understand what you're asking here.