As the title suggests really. Ive always thought DP was a little OTT with the anxiety he feels regarding his kids, he was divorced when we met yet it took him ages to even tell his kids about me never mind let me meet them as he was so scared about what they'd think (they were 16 and 17 at the time so hardly babies!). He also went on constantly about the effect divorce has on kids and how guilty he felt. I was sympathetic for a while but after a year of this I adopted the attitude "well christ, go back to her then!" because it seemed to me that all he ever thought about was his old like and how gutted he was that it fell apart. He never showed any enthusiasm for our relationship/lives. Anyway, time went on, we moved in together, I get on great with his kids (who had no issues accepting me once they finally got the chance to!) and on the surface he's great with my kids.
But for the past few months a few comments from him have made me think that a) he resents my kids/living with someone elses kids and b) he likes to take his guilt out on me. For instance DS's bike got stolen and so I mentioned to DP that I'd need to get him a 2nd hand cheap bike for his paper-round (of which ds would pay us back for). DP then starts whining about how much time and effort he put into the last bike and said "ffs I do more for your kids then I do for my own"
He often says "I don't see my kids now! but you don't care about that, you probably prefer it!" (he has the kids every weekend and as I've said, we get on great so this is just bullshit).
The other night he asked me to put on a move for us to watch, we sat down nice and cosy to watch it with a glass of wine and then he buggers off into the other room. I leave it for half hour and then go to find him sat on ebay on the pc. I say "what you doing?? we're supposed to be watching this film!?" and he replies "oh well I was doing something for my ds but no you're right, all my attention should be on you and I'll just ignore my kids." 
Another one was "well I won't be taking my kids on holiday this year as you wanted to go to Thailand so we won't be able to afford anything else." - My original idea was that we take HIS kids on holiday, he decided against this. I think he would have been happier if I said I didn't mind not going on holiday myself so he could just take his kids away by himself.
Anyway I'm rambling. I just get the impression that he resents the time he spends with me and my kids and just wishes he could have his "real" family back :(