Aibu to call them a cheater?

[SassyTee]

Not sure if I'm overreacting.
Happy to hear from anyone who's been somewhere similar, or be told I'm being too sensitive, or I'm doing the right thing.

Been with partner for 2 years. Good relationship, few ups and downs as we have both been dealing with highly stressful personal situations so have let the stress get the better of us and taken it out on each other a couple of times, but always communicate well and learn how to do better in future. Overall it has been a respectful, kind, safe, loving relationship. Or so I thought.

One of those stressful personal situations was recent, and we had a few cross words. As mentioned, we talked about it after we'd cooled down and was OK.
Well 2 weeks after that, partner was staying over. Their phone kept buzzing so I picked it up just to check it wasn't their parents trying to get hold of them (1 parent is pretty unwell, never looked at their phone before or felt the need to, they've never been secretive over it and will leave it next to me when out the room etc). Anyway, what i saw was a newly downloaded dating app (I know it's new as it's one that has only recently been advertised so didn't exist until very recently).
I woke them up and confronted them about it, but they got mad at me for having their phone, grabbed it back and stormed out.

That was 3 weeks ago.
I have had all the professions of love, sorry, 'it was a stupid lack of judgment'. They say they saw the advertisement after we had the falling out 2 weeks prior and just clicked download, but then didn't bother setting up a profile and hadn't thought about it since.
I am of the opinion it's cheating and whether or not a profile was made, intent was there if only for that moment they sat and downloaded it.
They don't understand why I'm making such a big deal about it and why am I willing to throw away 2 mostly amazing years on their 1 moment of stupidity. I say they are the one who threw away 2 years the moment they decided to download an app.

It's been 3 weeks since I ended it after discovering the app.
I currently don't trust my own feelings or judgment as it's still quite fresh and I have other things going on at the moment causing me a lot of stress too.

Am I overreacting? Would you react the same?
Is it cheating, or not? Would you end what was overall a good relationship on this one occasion?

So i suppose, AIBU to call it cheating and dump them, or AINBU as it is definitely cheating.

You sound torn but also hoping people will day give him another chance, that's how it reads to me.

Thing is, with the trust gone, it will never be the same.
A partner who reaches out for others in a time of crisis, isn't someone you can truly rely on.

I'm not a fan of the casual LTB you find on here over little things that can be fixed, but trust is difficult to restore.

I have been in a relationship where I turned into a little detective after choosing to forgive something similar.

Truth is, you're then always on edge and can't fully trust them.

I'm also a believer of that if you forgive a cheat, they somehow see it as you being somewhat ok with it.

The sort that think it's ok as long as you don't find out.
Because they hide it well from you, they believe they're being respectful.

I’m sorry, it shows (at least) an intention to cheat. It sounds like you were arguing but not broken up when they did it.

Also red flag is that they didn’t immediately show you the app (or lack of it).

Once the trust has gone it’s gone I am afraid.
Even a half decent person knows that when someone starts ‘looking around’ it’s came over.
The partner here is upset about getting caught. There are things about OP that they will miss.
The age thing is interesting - it can be stressful trying to run careers and look after parents etc. But does it mean you cling on to the familiar because the thought of ever finding someone else feels too hard?
I know it’s not okay but I do understand how outside stresses can sometimes push colleagues together - very dangerous.
But to actually download an app it’s pretty blatant to me.