I’m sorry OP, but I think he is needling you into leaving ( or throwing him out, a better option).
if he wanted to maintain his marriage , he would have STFU after he saw your first reaction to his little disclosure. Instead he has ramped it up, telling you that she has the same feelings about him, that they hold hands…..when he gets home he will be telling you that they had a little cuddle in the stationery cupboard.
I think your DH is a coward. He could have told you that he is in love with this woman, and she returns his feelings. Then you could have started to plan , together or separately, what you are both going to do in the future. That’s not what he has done. I think he wants you to leave him, so he can pose as the poor deserted bloke abandoned by his unreasonable wife for an over reaction to his ‘confession’ of ‘a minor indiscretion.’ It’s not good.
A male friend once told me that he had got ‘close’ to a female colleague ( not as close as hand holding, though, and certainly not the declaration of intent). He knew he was a dickhead, and he was worried. I told him to tell the colleague that he liked and esteemed her, but he may have been giving out the wrong signals, and that he was committed to his wife. He did. I gather it was an ‘awkward’ conversation and the atmosphere was ‘ a bit frosty’ . He’s still married, though, and as far as I know, happily married.