I'm actually in one of these relationships. It's got like this because I'm generally a laid back and generous person in my relations with others. I'm also quite passive (WAS quite passive). I dont generally expect payback for things that I do for people - most people naturally have a sense of what is fair to give and take.
In my relationship with my partner Ive had to learn to be different. He is very physically lazy and, as he'll admit himself, quite selfish. When we were young and single this wasn't really an issue. Yes, I did most of the housework and cooking but it was very, very little. We both had free time to enjoy ourselves etc.
Once we had a child this became more of an issue, obviously. For the first year I did every fucking thing. Every night feed, every bedtime, all the housework, had no lie-ins, no money and no free time. His excuse was that he 'worked'. Obviously that was a big crock of bullshit. One of the reasons I didn't assert myself is that I didn't want to be one of those couples who was always keeping score - I wanted a natural balance of give and take.
I've since come to the conclusion that I'll never be able to have that relationship with my current partner. I've gotten very assertive about the divvying up of chores, bedtimes, free time etc. But I literally have to always be keeping an eye on what he's getting v what I'm getting because he will take the piss wherever he can. Its exhausting having to constantly assert my right to exist with the same rights he does- having to get into a major argument over things that should be minor -like who's turn it is to do the washing up etc and it pisses me off but actually I've become so much more self assured etc.
That is pretty much our only relationship issue though (I know its a biggie - there have been times I've thought very seriously about leaving, even though we enjoy each others company / humour and interests and are compatible in so many ways).
He still doesn't understand how easy he had it in our child's first year though. One day I'll make him understand what a dick he was.