It should be taken into account that many monsters are possibly magical
creations, and as such need not abide to the natural laws for the
procreation of macro-scale organisms.
For instance, a gelantinous cube is a type of ooze, oozes have no
intelligence score and a wisdom score of 1, from this we can infer that
oozes probably do not have sexual relations, they are sexless, and one
could argue that oozes, not being sentient, are nothing more than
minerals with a purpose.
The closest analogy that springs to mind is that of cellular mitosis,
in which case a given ooze would create offspring by splitting itself
into two equal parts, each part taking on an existence of its own.
Another option would to treat oozes not as living organisms, but as
mineral corrosives, animated by magic in some way, in this case we have
do not have no procreation, one could hypothezise that oozes, while not
being able to procreate, are able to increase their mass through some
acidic means.
For instance, you have 20 gallons of ooze, crawling through a dungeon,
it happens upon a hapless adventurer, which it promptly devoures.
Let's suppose the adventurer, with assorted gear weighs in at 250
pounds, let's also assume a pound/gallon ratio of 100/5, that is, for
every 100 pounds of mass the ooze devours, it gains 5 gallons of ooze.
We would now have 22.5 gallons of ooze creeping about.
Now let's examine another scenario, in which the adventurer encountered
is not so hapless. He sees the ooze in time and decides, for some
nefarious purpose, to scoop up some of it into his handy flask. He gets
2 gallons into his flask until he is forced to retreat. We now have 2
oozes, 18 gallons on the floor and 2 in the flask, each intent on
exploiting the laws of conversion detailed above.
Thus a possible system of procreation emerges for mineral oozes.
Adventurers need not always be involved, parts of an ooze may slide
down a hole and whatnot.