Welcome to SI, the best club nobody wants to join. There are some pinned posts at the top of the forum that you may find helpful. There is also the Healing Library, which is chock full of information and includes a list of the acronyms we use.
For me, an engagement is a trial run for M and entails monogamy. Unfortunately, your WBF (wayward boyfriend) has failed that test. Many will come along and say that if you are not married and have no kids that you may want to cut your losses and leave the relationship. You two should still be in the honeymoon phase of your relationship and he couldn't stay faithful. Once you add other responsibilities, things get more complicated.
There is a book called How to Help Your Spouse Heal from Your Affair by Linda MacDonald that helps the wayward understand a little bit of the devastation they have heaped on their intimate partner. I suggest that you read it, too.
they kissed and fooled around
Just because they didn't have penis in vagina (PIV) sex doesn't mean he wasn't unfaithful. To me, kissing and fooling around is a sex act.
I think if he could have got them to meet he would have went
I agree with you. I think talking to other women on apps was a fishing trip and he was trying to see what he could get with his bait.
Infidelity is the worst pain imaginable. If you can find a betrayal trauma specialist, especially one that deals with infidelity, I would suggest IC (individual counseling).
BW M 34years, Dday 1: March 2018, Dday 2: August 2019, D final 2/25/21