I dont know if they are harder than girls because I have had 3 boys and no girls... I can say, I have 1 boy who is fixed and never marks. I have another boy who is not fixed and does mark. I keep a diaper on him. Im going to get him fixed soon though so Im sure the marking will stop once hes fixed.
Oh yeah and my boys never hump anything. Once in a blue moon Dexter will climb on Fletcher (the fixed boy climbs on the not fixed boy) and becomes a lazy 2 pump chump. (dominance I think but its only like 2x then he goes away) LOL Other than that, no humping in my house!
I love the little gentlemen! They are the greatest!

Good Luck with your search!