Nope. Because I am selfish... but fair. But selfish. I married you, not your kids. One day you and I are all there is left. The kids will all be gone and doing their own things.
... this is what my DH said to me.(Except for the selfish bit. That's me!)

I don't know, I just always assumed I came first. I never treated my SD as an outsider, and I would have taken a bullet for her when she was young, but as far as relationships go, I knew I came first. Admittedly, I might have just assumed that but my husband seemed to go along, so......

#1 Priority - your relationship with your partner
#1 Responsibility - kids

When we spoke to our Counsellor (whose had 17 yrs experience working with blended families), she explained this to us:

Relationships can be much like an onion. When the core of the onion is rotten, usually the entire thing gets thrown out.

Core of onion: Couple in a relationship ONLY.
1st layer - kids
2nd layer - immediate family (e.g.Grandparents)
3rd layer - extended family (Cousins etc)
4th layer - friends
5th layer - acquaintances

..and so on.. each layer also represents a boundary. She explained that a lot of problems stemmed when others cross their boundaaries. Perfect example of this is, earlier in our relationship, SO would put his daughter first - in other words, if she was too upset for him to visit me, he wouldn't. The Counsellor explained that by doing this, SO was pushing me to an outer layer and keeping himself and SO in the core. Relationship had 99% chance of failing if he continued to do this.

This explanation was like a light bulb going off in SO's brain. He understood and started to fix things. Mind you, SD is still fighting to be in the core and refuses to acknowledge our relationship. She hasn't visited, has a nasty attitude and demands that SO needs to respect HER. He's standing his ground and we haven't seen her since Sept 2013. Until she can accept that her Father is happy and respects that, she can stay well away from us. She cannot control him and neither can BM.

I am so grateful to the Counsellor for helping us understand this.

SO and I were both raised with the belief that spouses come first.
Kids are going to grow up, move out and get their own lives and families, you are with your spouse forever.

As evidenced by the brilliant signature tag line I borrowed from Dtzyblnd, I too agree the parental relationship comes first. It's like when you're on an airplane and the fight attendants demonstrate using the oxygen air mask in an emergency and they always say "SECURE YOURS FIRST BEFORE HELPING OTHERS" - basically because you're no good to be helpful or healthy for anyone else if you ignore yourself. Secure the relationship with your spouse first before prioritizing kids' wants (not their needs, which should be met along with your own).

I'm always surprised when this concept (adult relationship first, kids second) is new to someone. And I lol when someone argues with me that I'm wrong. Usually it's the people with an openly dysfunctional marriage and rotten children.