Nroug7: A door to a house is vital, however, if one does not build stairs then the door is pointless. In order for the door to be useful, stairs must follow.
"a house" = a closed space that provides an "inside"
"a door" = something that provides entry to said "inside"
"vital" = necessary for life
Now, what's an "inside" that's closely related to life? That's easy: a womb. And what is providing entry to a womb? That's easy too: a vagina.
"pointless" = not fulfilling its purpose
"stairs" = something pointing upwards
Now, if the purpose of a womb is to create life, then what kind of upward-pointing object is necessary to ave it fulfill that purpose? Well, that's easy too.
"follow" = to move toward something
In order to help the "door" fulfilling its purpose, the "stairs" must "follow". In order for the vagina to fulfill its purpose (i.e. functioning as a door to the life-creating womb), a penis must move inside it.
So, in other words, the meaning of the encrypted sentence is:
"You need to move a boner for successful conception."
There is also a sexist undertone to the riddle as it reduces sexual organs to only have the purpose of providing a means for procreation.
And actually, and perhaps somewhat underhandedly, the above solution is a riddle in itself. It's meaning is: "We really don't have enough information to go by, and based on the current lack of useful info, we can really interpret _anything_ into the riddle - see?".
;-p