Wife cheating

So today my bf (Boyfriend) found out his wife is cheating on him with one of his friends and he's really upset.

He has been suspicious of her for a while but today she forgot her phone at home and she received some text messages from one of his friends. When he went on her he came across messages where she has been sexting this guy and sending him nudes.

I really feel bad for him.

I know this is hard to understand but he loves both of us and I don't blame him. His wife is really beautiful and interesting but sometimes she's a little emotionally unstable and things between us just happened since he has always confined in me when things between him and her aren't going great.

I've never stood between them and I know I'll probably never be more than a "side chick" but I know that he loves us both very much. He has always been very honest with me about everything.

I get why some ladies on here is upset, but we never planned for this to happen. He has often told me if polygamy was legal he would marry us both but it isn't.

I have tried to walk away from him but it's not easy if we both love each other. He's always tried to be there for his wife and really dies take good care of her but it's like she doesn't realise just how much he loves her.

By the way why do people frown on polygamy today, it was a normal part of life hundreds of years ago.