I realise that some posters can't imagine totting things up and not completely sharing finances, but everyone's circs are different.
For eg. in my first marriage, we met at 16 & 18, and neither of us had 2 pennies to rub together. We got married, had kids, H earned more, but we just shared everything.
In my current marriage, we met later in life (late 30's). I had 2 kids and a house. He had no kids, and was renting. Having been through a divorce and splitting assets etc, there was no way I was going to put another man on my mortgage. Also, he is a spender and I'm a saver, and I've never fancied pooling our money.
Op, I totally understand your frustrations. Me and DH earn a similar amount, but he is such a spender (he has 3 bloody cars), that we've ended up in the situation whereby I have good savings and he has a bit of debt. That's not my problem. However, because of this, I often find myself paying for things that we would have split if I didn't have this nagging feeling of being better off than him. So I pay for take-aways, meals out etc, plus repairs to the house and stuff. Like your DP, he would order expensive things without a care in the world, but I'm paying for it! He does work FT though and have a pension.
I would be very worried about your DP's long term prospects, as it sounds like a frugal retirement. How old is he?
I think what bothers me and you, is their lackadaisical attitude to money, because that's the very reason why they aren't in good shape financially, but that directly impacts us, because we have to pick up the slack.
My DH ordered 2 insulated cups for an upcoming holiday - £70! They are great, he says. Well, that may well be, but A) we already have insulated cups, and B) the reason I'm always picking up the bill in restaurants and for treats, is because I have savings and you don't, but this is why you don't! The crazy spending. (His reason for buying new cups was that these latest ones have detachable stems - aargh).
I'm not sure what you can do - but I think I would ask him to work 4 days a week, and start a pension (assuming he is young enough to make it count).