My DH made it quite clear our relationship/marriage was for US. However, there are a number of people here who feel like you do. I'm sorry.

Yes and no. My husband valued marriage more than his previous spouse and when I met him he wanted to show his kids that marriage could work and was important. At the time I was happy to find someone dedicated to marriage and his kids. But sometimes, especially at first, this led to bullying because I was the new one in the mix. I felt unloved and that he married me, at least in part, just to prove a point to his kids or flaunt me to his ex. Soon, I realized other facts such as he was closer with his kids than he was with me, largely because he had known them longer. It felt like an uphill battle. 

There is hope. I started to focus on things that actually worked. For example, I made the kids important to me and called them "my kids", treating them as if I had the same love for them as a natural parent  - even though I didn't have the same privileges as a natural parent. My husband responded because he loved them too. I also focused on strengthening my marriage through things like dates and talking if there was a conflict or even if I just had something weighing on my mind. I would take him to another room and talk with him calmly and reasonably, with as much love and patience as possible. If you do this, be sure to do it as close to the time of conflict as appropriate - the change of scenery helps as does the brief time to calm down while you are going there. In short, strengthen your communication as much as possible. At first my husband thought I was going to scold him, but more often than not these discussions ended in laughter. My kids hate when we do this, but now my husband defends these talks and stands up to the kids for me. My marriage still has bumps but I now feel my husband is more dedicated to me and our marriage than the idea of showing off to the kids.

If I ever felt that I was there just to put a check in the right box for someone.... or was married to fulfill the "mother role".. I would be out.  the only reason you marry is to have a life partner... does not mean you are going to be mom to their kids necessarily.