It's ok to a point when your married, but not before. I am a step-mother to two children. When I only saw them every other weekend, it was to spend time together and have fun, period. But when they moved in with us things change. You are a step-parent, you need to make your feelings known and make sure the kids follow the rules while in your home, no matter if it is for the weekend or full time. The best thing that every happaned to my marriage was my DH's kids moving in with us, not just for us but for them. I taught them things that their mother couldn't bother with and brought them up as well adjusted young adults. Everyone's situation is different though. Sometimes a step-parent does not give the rest of us step-parents a good name!! But then again, step-kids can be the same way.
You have to do what is right for you family. I have read people saying or quoting what Dr. Phil says, well Dr. Phil isn't always right. You can't let your SO (when your married) sit on the back burner and let your kids run your lives. He in my opinion needs to take an active role, especially since the kids real father is MIA.
Ok I'm sorry I've wrote a book here, but honestly I have very strong opoinions on this question since I've been there done that. Unless your in the situation you don't know. You can have an opinion on it, for sure, but it doesn't mean it's right.
I wish you and your SO and the kids all the luck in the world. But just remember, if he's going to be you DH one day, don't make him resent you because of the kids and vice versa.
J. in Macomb, MI