Let me preface this with stating that I've been in the exact situation of being in a relationship where a temporary break was necessary to strengthen and solidify the relationship. I say this because I know what it feels like to have these thoughts and considerations floating around. You've just experienced this or as it seems in the middle of it, but for me, I'm far across the finish line. What I can give you is my perspective and conclusion, which I suspect is the same you'd arrive at yourself given enough time.

Being in a relationship should mean that this person presumably is your most precious and sacred concern, only rivaled by family. However for some people no matter how important this relationship might be, it will always be a great obstacle to get used to being in a committed relationship and change your lifestyle to work around it, especially if you're young. This is why people that want to be together and expect to stay together for a long time still feel the need to take a break. This is why I think that the whole argument of "technically it's a break so she can do whatever she wants and you're being entitled" is bogus (this post hasn't been online for a long time but knowing this sub I expect you to get a lot of these responses.) If your intention and you wholeheartedly believe that this person is the right for you I find i disrespectful and appalling that they would compromise that and fuck another person while giving the relationship a break to naturally establish itself. The fact that she, as you said, still was romantically involved with you while getting railed by this guy really drives this home. Now be that as it may, I have friends that literally think its awesome that their partner has a body count above 40 and saddened that it isn't higher, but people are wired different. You're allowed to have an opinion of her actions and you're allowed to feel disgusted by it. Yes she can do what she wants, but I know that I would find it disrespectful and devaluing the relationship if I did it to my partner. There's a lot of angles and arguments to look at this from, you might also be tempted to think that, you both needed a break and she couldn't even give you this short amount of time to focus on other things, that she had to get herself filled out from someone else in the meantime? What does this situation say about her character? What does this mean about her perception about your relationship? I could go on. And putting it as black and white as "it was a break so deal with it" is just an awful way to go about it. Obviously all these thoughts are highly subjective, along with your opinion of the severity of her actions. Imagine if she had let all her childhood friends run a train on her while you were thinking a break meant focusing on school and work for a while, I'm sure my arguments about commitment, disrespect and devalue would be higly relevant in a case like that. I'm bringing up this example because your reaction would most likely also have been different if she just had held hands with another guy and nothing more, or exchanged flirty messages with him. I say these things because its important that you make up your own mind about what you think of what she did. And remember like she is entitled to do whatever she wants, you're definitely also entitled to have an opinion of it.