I know what you mean. Good dads want their DC to be happy and adapt to change in a realistic way, and good partners/boyfriends/husbands want their new partners to feel part of the new unit, to bond their children in their own time, to keep doing the bulk of the parenting and have the step parent enjoy the good fun bits.
Where it quickly goes wrong is when men assume their new partner will step into unconditional-mum-love immediately, whether they have and want DC of their own or not, do the bulk of crappy housekeeping/parenting stuff, then step aside knowing instinctively when to make themselves scarce to accommodate what the DC and/ex want.
I've been very lucky with a considerate DH and wonderful DSC who've accepted me and seem to value the part I play in their lives. But even then it's been incredibly difficult at times! I've also learnt that I make my own decisions about how much I want to be involved with, while trying to balance everyones needs. I wash clothes, make endless meals, arrange days out and holidays, read stories, patch up knees, dole out calpol, have time with them alone so the other can have time with DH. But it's my choice and I get the benefits of lovely, kind children and a happy husband. He'd cope fine without me. He's a brilliant dad. He'd happily do it all alone. He's let me find my own way of being in our family.
I don't understand why some women allow themselves to end up as general pot washer, cook, cleaner, babysitter, dogsbody, while their partner sits back and doesn't parent his children, clean up their mess, pay for them, discipline them when needed. But it's easy to say how these women should be different, when it's their partners/the parent who should be better!
Men who were useless husbands and fathers aren't going to be any better with a new partner, though some men might become better parents when they split with the other parent purely out of necessity as they're doing it alone.
In a way it's easier to get together with a man who's parented alone as he's capable of doing it on his own and hasn't got used to having another adult to share it with. But you also hear plenty of stories where post split either/both parents doing it alone develops dynamics with their children which then doesn't easily allow another adult in. Epic Disney parenting where they only want the good bits, won't deal with bad behaviour, let the DC rule the roost to the exclusion of anyone else in the household.
What advice would you give men/parents on how to make life easier for the step parent coming in?