TheLadyRadishes said Yes, on the one hand it's good to be honest. On the other, don't let that make you think he's a lovely, poor tortured bloke just because he's fessing up and indulging in a lot of soul-baring about how he loves you both. That does NOT make what he has done OK and if you look at the bigger picture, he has form for buggering off to claim his freedom in a thoroughly entitled manner.
I agree, and I would add that he thinks he loves you both. But he doesn't. That's not love. He fancies you both. He finds you both useful. He finds you both good company. But that's all about him, isn't it? All about how his life can be enhanced by having you both in it. And how he can benefit by playing you off against each other to win his affection.
He doesn't appear to really care about either of you as people with feelings, opinions, ambitions of your own. If he loved you he wouldn't have cheated on you and then acted like it was nothing, that decisions about the future of the relationship are his alone to make. If he loved her then he wouldn't be jeopardising her career, lying to her and would make himself available to be with her. Instead he's messing you both about, trying to figure out what's in his best interests. The only person he loves in all this is himself.
This has all come as a terrible shock to you. You need time and space to grieve the relationship and husband you thought you had. It didn't exist because secretly he was someone else, cheating on you behind your back whilst you were raising his child, sacrificing your old career and trying to build a new one. It maybe feels like person you thought you married has disappeared, and in place of him is his identical, but much less admirable twin. This twin would cheat on the mother of his young child, behave unprofessionally at work potentially risking his job, pretend to be hard at work whilst carrying out an affair, spending family money and/or his employer's money on the OW, blame you for working, blame you for SAH (as he wanted), for having children (that he wanted) etc.
I understand why you would grieve the man you lost when you discovered this. But going forward, the only man available is the cheating scumbag version. Of course you think you love him, because he looks and feels like the man you lost, but he is never going to be that first man again, because he never was. He doesn't think there is anything wrong with a drunken one night stand, or a couple of shags after that. Remember that, he is telling you who he is. Believe him.
What happens after all this torture, this forgiving him and rebuilding the relationship if he decides to leave anyway? He thinks its his choice to make and has already said that he thinks he'll resent you for spoiling his fun.
Definitely speak to a solicitor so that you know what the practicalities would be if the relationship does end. Its better to have that knowledge even if you don't use it, in case mr. unreliable disappears in a month/3/6 months time. Know your options 