That's not always true. You're coming from a biological point of view. Please take into consideration the work a "non biological parent (step parent)" takes on when entering a relationship with a person with kids. They (bio parents) feel entitled. The father needs to teach his maturing 9 yr old daughter to sleep in her own bed. I told my bf tonight that I will no longer sleep over until he and his aging daughter have their sleeping arrangements worked out....he laughed at me and said no matter what, he will never tell her no. Defensive parents won't be able to see other points of views. I think it's harder for people without kids to be in relationships with people who have kids. I give my all but it's never enough when it comes to people and their children...NO matter how good I am to them.
I think the more you make a fuss the more likely the behaviour will remain. He is her father and this territorial stuff is fairly normal. Do you have children? I still have our kids occasionally jump into bed with us. (Admittedly it is not all the time).. It is normal don't make a big deal out of it or you could end up losing your boyfriend. She will get used to you with time. Good Luck