You expected him to act married, because he was married to your mother, who was not dead. You had every right to care.
At this point, though, I think you have to ask yourself how you would expect him to treat you if you had made your own adult decisions that he thought were wrong-headed or rationalizations of immorality.
He can’t go back and un-do what he did. The woman he intends to marry did not make your mother sick, she didn’t have anything to do with her death. Your father is free to marry, now.
I think the way to go is to presume that any sin he committed is something he could have repented of and confessed and been absolved of, all without your knowledge. If he tries to convince you he did the right thing, correct him. Otherwise, stay in today and let yesterday go. Your mother will have, by now. She is beyond holding a grudge, you are not showing loyalty by holding onto this offense against her, and for her sake and your own you ought to get beyond that past offense against your trust, as well. That is a process, that letting go of ill will over a past offense that cannot be un-done, not just something you decide to do. It will be better for you if you work in that direction, though.
I think you say, “I am still grieving Mom, Dad. You may be ready to move on, but I am not ready to talk about this. You’re going to need to leave me alone about it for awhile. I hope you understand.” If he doesn’t, that is his issue. Do not offer any defense for your grief. You don’t owe him one and, frankly, nothing good will come of it. You may say something you will later wish you had not bluntly put into words. He undoubtedly has very difficult emotions to cope with, too. You’re unlikely to say anything that is news to him.
As for the wedding, play that by ear. Today’s worries are sufficient for today. If they are both free to marry, it is not an invalid wedding, but because of the circumstances of the “courtship,” you might have to tell your dad you would be bad company that day. Wait and see and reserve the right to wait on your decision, if he asks. (I kind of doubt that he will.)
In other words, for today reserve the right to say nothing, if you have nothing to say that is likely to help matters. Do not feel you owe any apologies for that and remember that sometimes giving reasons does nothing but invite rebuttals. Point-counter-point can get very ugly when emotions are raw, and you can choose not to go there.
