I agree 100% with specialmom and I had the same initial reaction that she did. If he truly loved you and wanted to be with you, he would be. They're not married...and obviously they haven't taken that step for a reason...so if he wanted to leave, he could. Children make it more complicated yes, but I still believe that he could leave her if he truly wanted to.
You deserve a man who will love you without distractions and his children deserve a father who is not deceiving their mother, even if he is no longer with her. I would set a time frame like specialmom said and see what comes of it...but be prepared for things not to go the way that you might want them to. But either way, you would no longer be stuck in the middle wondering if you were being used or not.
You deserve more than to be "the other woman"..so stand up for yourself. You may lose him, but then at least you know that it was never real and you can move on and start again. Who knows, he may leave her and come to you and you can start a real relationship...but be forewarned that there are a lot of problems with relationships that start with a partner who has been unfaithful. It's definitely possible though.
Good luck either way
specialmom said it perfectly. Here's my thing and I'm not trying to be judgmental here, but I am a firm believer in the philosphy of do unto others as you would want done unto you. How would you feel if you had a boyfriend, in which you had children with, and he was sleeping with your friend? Not only is his character in question, but yours is as well. They aren't just boyfriend and girlfriend, they have children who will be affected by this situation just as much. If he doesn't even think about them, you at least should. I'm sure you know them, since you are her friend. The fact that you two flaunt this relationship, if that's what you call it, infront of mutual friends, is cruel. This girl will not only be, hurt but humiliated that everyone knew this was going on and she knew nothing. It's a double betrayal. You need to understand that if he was so invested in you, then he would've done the right thing and left his girlfriend first. The fact that he hasn't leads me to believe, he's comfortable with having her be his main girlfriend and have you on the side. Let's say they do break up and you stay with him, won't you worry about his faithfulness with you? I mean, if he could cheat on the mother of his children, then what would be the difference with you? Think about it! Oh and to answer your question of jealousy, well that's a control factor. He's trying to keep you just where he wants you. He makes it seem as if he cares so much so that you won't be tempted to go anywhere. Trust me on that one.
Well, the thing is . . . that being in love does not make things right or work out. Love does not conquer all. It can cloud our vision from making right and healthy choices for yourself. You are a young woman and this is not the best way to start you experience with love relationships.
If he is in love with you and wants to be with you and doesn't care who knows, why has he not left his girlfriend? I think you really need to think about that. I would also think about a man that has a girlfriend and children and starts a secondary relationship with his girlfriend's friend. That is not someone full of character. Love is not an answer to that question. I've been in love with a few different people and I did not let it cloud my vision of right and wrong. He is doing the wrong thing. You need to see him for what he is. He is cheating.
You deserve a relationship that has the innocence that all relationships should start with. This one was tainted from the beginning. It is hard to make things work out long term after starting like this. The statistics are not favorable. I know this is you and I don't know you and this could be a relationship that is going to do just fine. But these are just things for you to think about.
If you have your heart set on following this through-------- well, you ask him to leave his girlfriend and give him a time frame. (as in a month). Then you start over as a dating couple that is legit. Be prepared for backlash. His kids may have an issue with you and some of your mutual friends with HER might not see this as a good thing. But if you want a relationship with him-------- he must first leave his girlfriend. Otherwise, you are just sleeping with him and no more. No matter what he tells you.
I sure hope that you've pursued some schooling and a job that allows you to be an independent woman. Don't get too lost in love that you forget yourself. good luck