Pre-loaded crossbows do not exist
Strictly speaking, there is no way to pre-load a crossbow. The “Weapon Properties” rules state (emphasis mine):
(…) Ammunition. You can use a weapon that has the ammunition property to make a ranged attack only if you have ammunition to fire from the weapon. Each time you attack with the weapon, you expend one piece of ammunition. Drawing the ammunition from a quiver, case, or other container is part of the attack (…)
There simply is no way to draw the ammunition for an attack other than during the attack; thus crossbows cannot be pre-loaded as the only time they are loaded is immediately before being fired.
Of course, this is possible using a made up action as the rules on “Actions in Combat” state:
(…) When you describe an action not detailed elsewhere in the rules, the DM tells you whether that action is possible and what kind of roll you need to make, if any, to determine success or failure. (…)
So you could use your action on the non-detailed action of loading a crossbow, though whether this can happen is up to the GM; as are the results and ramifications of allowing such an action.
The bit about slings merely requires that you have ammunition available
(…) If you use a weapon that has the ammunition property to make a melee attack, you treat the weapon as an improvised weapon (see “Improvised Weapons” later in the section). A sling must be loaded to deal any damage when used in this way. (…)
This merely clarifies that in order to use a sling to make an improvised melee attack, it must be loaded; and we know from the previous bit that slings are loaded when you attack with them. Thus, this is just stating that for slings in particular, you must have ammunition (and a free hand) available in order to use them to make improvised melee attacks.
What I’d do at my tables
I would allow characters to pre-load crossbows by using their action, though I would not have this make them ignore the Loading property (they could not use Extra Attack to fire two pre-loaded crossbows). Thus it effectively just gives them an extra hand to do something with if they wanted. This is very similar to what I actually do at my tables which is to basically ignore the drawing/stowing rules altogether and allow characters to draw/stow any number of weapons with each of their attacks.