Ok, the part about him finding her attractive, it's hurts to know that our partners wil find others attractive and be sexually attracted to others. But that's human nature.
However, he crossed the line by acting on the attraction by texting her and then went even further by asking for pics and then giving her compliments. And the poem, FFS. That suggests more than just an attraction. I can totally understand why you're hurting.
He's minimising things. He's angry because he doesn't want to have to apologise and because you now have 'power over him' but also because he's had to nip his little infatuation in the bud.
Why have you not mentioned seeing the poem? Perhaps, with him knowing you've seen it, he'll stop trying to pass it all off as 'just a few texts'.
He's not being fully open and honest with you right now about exactly what he did, the extent of his feelings/thoughts about OW and why he feels things could have come to that. And until you know the truth then you'll not know whether it's something you feel is possible to be worked through.
I think you perhaps need a firmer approach here and be honest about that poem too.