Also forgot to say, I have 2 boys both uncastrated, one who is 2 and one 8 months. My younger dog hasn't started scent marking yet, but I found that when Neo started to do this, then this was when he became properly toilet trained and the occassional accidents stopped.
I don't let Neo stop and scent mark everything when I have him on lead, he's only allowed to do it when I let him off for his free running. I don't find the scent marking on it's own reason enough to have him castrated.
Having 2 uncastrated males in the house may cause issues, I left an 18 month gap before getting another dog to help with any dominance issues. At this point in time things are fine. If we start to get fighting I will then review the situation. My eldest dog is very soft and we've not had any behavioural problems associated with him being intact.