My girlfriend didn't wait very long before introducing me to her kids, only a few weeks after we were officially dating. I go over all the time, and they're a very social family so the kids are used to people coming and going. They two youngest don't yet have any concept of romance, and the oldest at 13 understands that romantic relationships don't always last, and their parents' partners aren't someone to get too attached to. At the same time, my gf explained very early that her kids already have three parents and they don't want or need any more, so I always just act friendly with the kids, but not at all parent-like.
I don't need labels to define me. They're sticky and I hate the glue they leave behind.