I was going to say pretty much the same thing GalaGirl said - it isn't something you "let" your partner do or not. After all, she is the owner and boss of her own body. It is something you agree to or not, something to negotiate, but it isn't up to you to "let them." That is their relationship, not yours. All you can do is state what you are and are not comfortable with and what the consequences would be if it happened (condoms for you and Mrs.? No more sex with Mrs.? Divorce? Seeing the paperwork of regular testing every three months?).
That being said, I think four months is wa-a-a-ay too soon for fluid bonding. It is a pretty serious step and, as I understand things, you all need clean test results not once but twice at least, six months apart, to get a true picture of the situation.
I think what's more important is to deal with the betrayal of trust. It was NOT cool that unprotected sex happened and you were informed afterward. You say there was "some hurt and anger" but I think that was a huge transgression and should not be overlooked. Can you really trust them to honor any request or boundary you have? And what have you done to rebuild the trust she had in you before you did something similar with J? I wonder if this was about getting you back for that.
Hot chick in the city.
Oh, oh, can't you see? Love is the drug for me.