Who is right here? honestly?

My bf has 3 children. I have none. I am 26 and about to graduate college and I have not had any children yet bc I do not want to accept the responsibility of them until I am financially stable. Well I cook dinner for his kids almost every night, I play with them constantly, I talk to them when they need to talk, and when they cry I run to them. I am their friend. Well my bf got upset with me the other day bc I did not want to stay at home and watch his children all day while he went to work when I had class and had other responsibilities of my own to take care off. He said since I am the gf of a man with 3 children it is my responsibility to do those things but I just disagree completely. My life does not revolve around his children. It may sound selfish, but my life revolves around my education right now. Am I wrong for not wanting to take that momish role? Are his kids really my responsibility, or am I right, and they are his and his ex wifes?