If there was a PA, I cannot see that marriage would be an option. I don't think I would ever be able to trust him. If there was a PA, that would be his third confirmed one.
So it sounds like you haven't been telling each other ILY for quite a while, has the intimacy also been lacking? Not sure what your sitch is, but I'm reminded of the chapter in 5 Love Languages that talks about how spouses can become paralyzed, both sitting with their love tanks on empty and wondering why their spouse doesn't fill it. So neither takes action, they just wait and wait and wait until the marriage crumbles. And even when that happens they still continue to wait, becoming more and more bitter the longer they wait. 5LL talks about how if you want your love tank filled you fill your spouse's FIRST and then they will be motivated to fill yours.
True (and also sad), but also would be inappropriate to do this now if there is indeed new infidelity going on here. Another DB basic ("pursuit.")
