(Spoilers Main) Bolton and boy lords

I am curious why does Roose Bolton say this phrase in particular

I will not live long enough to see new sons to manhood, and boy lords are the bane of any House. Walda will grieve to see them die, though.

According to Westerosi culture the main way to continue the line and ensure the success of the family is to have male heirs to continue the name since females almost always are left out of the chain of succession.

Not considering Bolt-on why exactly would he say this besides the fact that boy lords will fight each other for who will be on top but still many houses get along with many male heirs (I mean it is necessary not only preferable)?