I'm concerned about my gf of 9 months revealing that her and her ex had joint custody of his kid
Oh ok, this is definitely some big news to take.
I don't like surprises and I like to know what's wrong before making decisions about it.
That stinks that she revealed this to you so late in your relationship. Hopefully you guys are able to talk about this situation and figure out if it is something that you are willing to deal with.
It is obvious. Don't worry if they want to sleep together she will cheat on me and I'll dump her.
Well if she cheats on you then you should without a doubt dump her. Nobody should have to worry about their partner cheating on them.