This thread has worked out very strangely, I can see why some posters are astonished at what appears to be almost a level of support for the OW.
I don't think it's about that really.
OP, if you are still reading - my humble 2p worth. You have had a really unusually varied bag of responses here I think because there is something about the story of this affair which leaves a particularly nasty taste in the mouth.
From what you've said about this woman, she sounds pretty unhinged and desperate. Her behaviour has obviously been awful and there is never an excuse for getting involved with a married man - but reading between the lines, it sounds more than anything that your H is the predator - a man who started an affair with a clearly vulnerable person. Who (I would bet a lot of money) indulged her with promises of leaving you, having a life together. Who slept with her without using protection. That last says it all, you MUST know that. Do you honestly believe that this was the 'mistake' of an honest, loving husband who acted utterly out of character and is now 'traumatised'? I don't. It doesn't sound like it at all. He sounds a callous user with as little regard for his family and their health as for this woman now that the affair is over.
Do you believe that she was utterly different when he started the affair - really?? Do you believe that it was she who ensnared him, like some powerful, clever femme fatale? From the way things are now, he seems to be the one with the nous, no? He's certainly been successful (so far) in extricating himself from her and making it clear that he will use the law against her to make sure she is neutralised, while at the same time being extremely effective (from this cynic's point of view) in getting you to join with him in this. While you are on his side, she has no power to upset anything - he knows this. Except for - possibly - the card she has shown now. Which is why I think - and hope, for all your sakes - that she may not be pregnant and that this may indeed be a last desperate attempt to fight against being 'discarded' (as she sees it). I am not surprised that she didn't have a formal caution. It sounds more as if the police had a friendly word advising her to let this go.
The most upsetting thing though is the degree to which you are taking his side. Nothing good at all can come of it. They both did wrong, but only one person made promises of fidelity to you. And it isn't the one which you are demonising. Don't let the wool be pulled over your eyes any longer. Have a good think, because, whether she is pregnant or not, your H sounds like a totally accomplished shitbag. He will betray you again because your reaction has shown him that when the chips are down you will fight FOR him - mummy tiger - despite him throwing you and your relationship on the scrapheap.
Does this equate to me saying that no-one should ever forgive an affair - I don't know. I think for me that would be the case, but I do know that I would be unable to advise anyone to forgive where there had been unprotected sex. He put your health in danger. And I also know that him 'coming clean' because he could no longer stand the deceit etc. is overwhelmingly likely to be utter bullshit - he was about to be outed.
So. I think the unusual responses on this thread spring from the same sense of unease as I got, reading this. Even with you, his supporter, as the narrator - your H sounds like a nasty, nasty predator. Beware.