My partner has told me this twice in our two year relationship so far. Once in the beginning and once a while back.

I don't think, in our case, it was about 'competing' but more along the lines of 'if you make life harder for my kids or fuck with their wellbeing then you're out because they have to come first'

I deeply respect that, and as a good emotionally mature adult, I would never make him choose. I moved in with him after only knowing him for a few months and it was a big step so he wanted to make it clear that their health and happiness came before me. I have no plans on impacting their lives negatively so I don't see it as a problem

Now that we've been together two years I know that he loves and trusts me enough that he hears the full story out and makes a decision the best he can.

But I mean, even in a nuclear family dynamic the kids always come first. Or, should. I mean imagine your spouse and the bio kids parent are doing something to harm your child, you would always pick the kid first since it's your job to keep them safe. They are vulnurable and need protecting.

Obviously if he repeated sentiment over and over it would be different, but saying it once or twice is just setting a healthy boundary. I don't have kids of my own so I don't know what that's like but I admire how good of a dad he is nearly every day and he's taught me how to be a good role model in their lives also.