I think that where there is an inequality in the relationship, kids can put pressure on those cracks and widen them. So if you've been carrying the load pre-children, but that's been okay because it's only (say) an extra 3-4 hours on the weekends and manageable, and then the kids come along and suddenly you realise that you are doing 16 hour days and your partner isn't, you can no longer paper over that, and resentment breeds.
The separation of roles is also hard. If one parent stays at home fulltime and the other starts working long hours, suddenly you don't have a marriage of equals. I know that when I was at home, in grotty jeans and unwashed hair, I felt second class to the people still going to work in nice suits and buying their morning latte.
After a few months, my husband and I struggled to make dinner conversation; oh, what did you do today? Cleaned the house, settled our daughter for a nap, played with her, read her a book that she didn't care about, settled her for another nap, why are your eyes glazing over? And he'd tell me that he had an evening networking dinner coming up and I'd feel so resentful that he got to go out and have lovely drinks and conversation with adults and I didn't.
When I went back to work and he took over the SAHD role, he felt exactly the same way. I remember once I went out of a Sunday afternoon with some workmates, and he came to pick me up after a few hours, and he said he was so conscious, sitting there in this gorgeous house of our hostess, that there was baby sick on his jeans and he hadn't had a haircut in months, and there was me and my colleagues in fashionable clothes and straight shiny hair and sipping rose.
Having said all of that, I do think it's perfectly possible to be alright. We are fantastic as a couple still, and love each other madly, and adore our daughter and our life.
One of the ways we did that, though, was to be ruthless about splitting roles. We now spend exactly the same amount of time with our daughter each (and we try and alternate social occasions that require the other partner to 'babysit' to keep that even - he went out to dinner a couple of weeks ago, I've got a Mnet meetup tomorrow).
We also make a point to be interested in one another's day, we thank each other for nice gestures like cooking something fancy, and every now and then we ship our daughter off to her beloved Nana's for an afternoon so we can spend it in bed together. Not everything is about the children, you know?