Giggly, you may not notice an imbalance now, but you will possibly notice the difference in parenting if you have your own child. Because, if my kids were ungrateful, rude, didn't greet me as I walked in the door, didn't compliment me (or each other) or thank me for anything, constantly goaded each other in my prescence and spoke with these phrases 'I want', 'get me', 'I don't want/like', 'do this' 'put this...' it would either be WW3 or I would call them out on it, there would be an apology and it would not happen again. It's natural in a parent-child relationship that we guide and discipline and influence in positive ways.
However in a non-resident parent relationship, boy, anything goes... It takes a strong man to parent a child in that situation, my dp much as I love him is not able to do it. It causes all manner of problems with us and behaviours that just would not be accepted with my dc are with his. Imbalance, unfair. Chaos at EOW's.
What Happy says about the dregs is true. In my situation it has turned out to be more the emotional burden. My dp admitted recently that he is so worn down about the kids, so depressed about them and the constant hell he gets from the mother, that he does not have any time for my 'issues' and that I am to 'give him a break'. So it does not matter how his dc treat me, I am to just accept it. How's that for romantic?
OP it's not all doom and gloom, but the key here is your dp's behaviour, not so much the kids. He must be a parent and your partner at the same time. Not slave to the kids, not you on the bottom rung of the ladder. You two, side by side.
P.S. I'm sure you would be a great mum one day, with your own kids, it's pretty easy. x