Am I just being selfish?

I (36f) love my boyfriend (36m) very much and I plan to marry him someday. He has a 13yo son, whereas I am child free. Last year we bought a house together, his son visits most weekends and everything was fine. However recently the custody arrangement has changed, and he is now with us 90% of the time. The rational part of me wants to graciously accept this situation and support my boyfriend as ultimately it is something he just has to do. I keep telling myself that I plan to spend my life with this man, and in future I'm sure we will have to support each other with everything life throws at us, and this is just one of those things. I should also point out that SS is really no trouble - he is quiet and well behaved, and I am very much a nacho stepparent and nobody puts pressure on me to take any more responsibility. HOWEVER the selfish, irrational immature part of me just doesn't like it!!!! I feel that it's MY home and I just don't want someone else's child living there!! Am I just being selfish and unreasonable?? Does anyone else feel similar??