Wow, the fact that he's married to her should help you forgive and realise it wasn't just a stupid mistake. He loves her, not your mum. Bit of a rubbish thing for someone to do (cheat) of course but he is happy now and not lying to your mum.
My mum cheated on my dad (emotionally or physically or both, still don't know) and she is getting married to him next year. It still hurts my dad but they're adults and they can both make their own decisions. Its not going to affect my feelings about my mum.
I'm suprised at the amount of people who hate their dads! I don't have the best relationship with my dad, mainly because we usually have nothing to talk about.. but I can't imagine hating him. It'd take a lot.