How are you defining single parent? A parent that is not dating anyone or a parent that is raising children without their father? I think of it as the latter.
If your boyfriend has stepped in and is helping you raise your children, then I agree classifying yourself as a single parent wouldn't be correct. But, if he's not yet stepped into that role of father/supporter...if you are the one that takes responsibility for staying home when they are sick, finances and gets them to activities, makes all vital decisions about their schooling, house rules, medical care, etc, then you are still a single parent.
That said, I do agree with previous posters. You don't necessarily have to define it. Those that are close to you know your status with your children, their father and your boyfriend. Others, like your boss and their teacher, etc, may need to see you as a 'single parent' for the purposes of expectation when it comes to you missing work to care for them, calling you for parent-teacher conferences/emergencies, etc.