Who said marriage is required for an heir. The premise of the Rogue Trader character from the campaign I plan to run is that his mother was the Rogue Trader and his father a pirate captain who she had done buisiness with and had a romance with.
I agree with Varnias, I dont think that arranged marriage is necissarily the norm. I think that it is possible for Rogue Trader houses to go down that road but I can imagine that it is very much up to the individual family.
Kaihlik