I don't actually think this is unusual. I'm sure there's been research showing that older widowers move on really quickly and get remarried really quickly.
I think some older men want a wife to look after them (it's a generational thing which is hopefully on its way out!) and some just struggle with the loss of identity of no longer being someone's husband. To you it's just a legal contact they don't need. But for some men, it's about getting their identity back, and feeling more normal is worth it to them.
And as the OP's partner is also no spring chicken, it's entirely possible she also has strong enough feelings about the label for the legal consequences to be secondary. Not being married is quite normal among younger generations. For most people in their 80s, being married is the sign of a committed relationship, and it feels weird having a boyfriend or girlfriend when you're definitely no longer a boy or a girl. Whether or not you agree, you can't make someone feel the way you'd like them to.
It's worth checking whether a man remarrying in this situation understands:
- The new wife will automatically be assumed to be the one making decisions re medical and funeral decisions. (Even if you have legal paperwork drawn up to say, actually, it's you, you'll have to override that and it might be too late.)
- If there's no will in place, the new wife will get everything.
- He might love his new wife and his children, but at this stage in life, he can't expect them to love each other. They're well past stepparent/stepchild ages. He needs to be clear with his wishes upfront rather than leave it to them to work it out after his death. That would just be cowardly and cruel.
- There may be implications over funding his care. The OP talks about health issues - is it more important to him to ensure his new wife is looked after, his kids are looked after, or that he gets his first choice of care home? It's not selfish to want to spend his and his first wife's accumulated wealth on ensuring his final days are as comfortable as possible.
If he knows the ramifications, if he's mentally fit... let him crack on. I'm of an age now where I will never be with someone for 50 years, so I can't imagine what it's like to suddenly be on your own after that length of time. If being someone's husband again is what makes him happy, I wouldn't want to get in the way of that.
Life is short. You take your happiness while you can.
It's likely you'll outlive your father. Think about how you'd like your last years together to be, and how you'd like his last years to be. The answer might well be with him married to this woman, and you spending time with her.
Having already lost a parent, you know all about grief, the what ifs and the regrets. If you fell out with your father over this and didn't speak to him again, what would that mean to you?
Yes, do think about what your mum would have wanted. But the casting vote goes to what your surviving parent wants. Because that's the parent who was left behind and is trying to find a way of coping.
PS My sincere condolences for your loss. I know it was 5 years ago, but it's a big loss, and I think anyone seeing a parent remarry would have all kinds of complicated feelings brought up in this situation. Be gentle with yourself, and be mindful you're not shoving all your feelings about everything up into a ball and chucking those around at any moving target.
No one makes their best decisions when they are struck by grief, and hearing about your dad getting a new wife is bound to make you have all kinds of thoughts about his first wife, and how much you miss her. I really am very sorry sorry you lost your mum, and didn't get the time you deserved to have with her.