So there’s been a bit of a discussion around the gender distribution of the monsters in the 5E Monster Manual. More research is always better, so I did some counting myself.
To cut right to the bottom line:
Total Monster Illustrations: 277
Asexual Illustrations: 221 (80%)
Males Illustrated: 37 (13%)
Females Illustrated: 19 (7%)
Ergo, changing 9 male illustrations to female illustrations, or 3% of the total illustrations, would result in gender parity.
My data and comments follow. It is, by its nature, often a judgment call, and I can appreciate that others may have different judgments. Maybe it’s obvious that flumphs reproduce by fission and I just missed it. Alas, I’ve done my best.
Sexual Biology, Depicted Ambiguously 
Dinosaur, Tyrannosaurs Rex
Dragon, Ancient Black
Dragon, Ancient Blue
Dragon, Ancient Green
Dragon, Ancient Red
Dragon, Ancient White
Dragon, Ancient Brass
Dragon, Ancient Bronze
Dragon, Bronze Wyrmling
Dragon, Ancient Copper
Dragon, Ancient Gold
Dragon, Ancient Silver
Giant Fire Beetle
Swarm of Bats
Ambiguous or Unknown Biology, Depicted Ambiguously 
Blight, Needle Blight 
Blight, Twig Blight
Blight, Vine Blight
Fungi, Gas Spore 
Fungi, Violet Fungus
Ooze, Black Pudding
Ooze, Gelatinous Cube
Ooze, Gray Ooze
Ooze, Ochre Jelly
Asexual, Depicted Ambiguously 
Demon, Balor 
Demon, Shadow Demon
Devil, Barbed Devil
Devil, Bone Devil
Devil, Horned Devil
Devil, Ice Devil
Devil, Pit Fiend
Devil, Spined Devil
Asexual, Depicted Asexual 
Animated Object, Animated Armor
Animated Object, Flying Sword
Animated Object, Rug of Smothering
Beholder, Death Tyrant
Salamander, Fire Snake
Sexual, Depicted as Male 
Elf, Drow Mage
NPC, Bandit Captain
NPC, Cult Fanatic
Asexual but Traditionally Depicted as Female, Depicted as Female 
Asexual, Depicted as Male 
Devil, Chain Devil
Asexual but Traditionally Depicted as Male, Depicted as Male 
Devil, Bearded Devil
Sexual but Traditionally Depicted as Female, Depicted as Female 
Asexual, Depicted as Female 
Sexual but Traditionally Depicted as Male, Depicted as Male 
Sexual, Depicted as Female 
Ambiguous or Unknown Biology, Depicted as Male 
Ambiguous or Unknown Biology, Depicted as Female 
Asexual but Traditionally Depicted as Male, Depicted Ambiguously 
Sexual but Traditionally Depicted as Female, Depicted Ambiguously 
Sexual but Traditionally Depicted as Male, Depicted Ambiguously 
 Aboleths have been around a very long time, and “they never die”, and the average campaign setting is not knee-deep in aboleths; together, this implies that aboleths probably either cannot reproduce at all or reproduce in some manner very different from standard biologies.
 The undead in general are a bit problematic under this sort of analysis. They generally either cannot reproduce themselves (zombies, for example) or reproduce in a broadly asexual fashion (wights, perhaps.) Still, they’re generally formed from the corpse of a sexual creature, so there’s that to consider. Finally, a monster like an undead skeleton might be depicted as the remains of a sexual corpse, but I personally do not have the expertise to identify the sexual dimorphism present in the human skeleton. I’ve done the best I can with the undead, but there’s a certain amount of inherent arbitrariness in how I’ve classifed them and their depictions.
 Tree-related-monsters are a bit of a challenge. Some trees have distinct male and female individuals, such as poplars. Other trees do not.
 Cambions have the fiend type, but they also seem to be half-mortal, so I’m not sure what their reproductive capabilities might be.
 Somewhat tempting to classify the centaur as ‘traditionally male’, yet female centaurs go back to at least 400 BC, so I’m not convinced it’s a strong tradition.
 Probably chimerae have two sexes, but the possibility of them being strictly-hermaphroditic is too tempting to sweep aside.
 The various Cyclopes of myth are invariably male, but it seems as likely as not that the D&D monsters described here are, like others of the giant type, sexual.
 If I was arbitrary with the undead, it’s ten times worse with demons, devils, yugoloths, and all the rest of these extra-planar non-biological entities. A few are obvious but many are not; I’ve done my best, but I certainly hold no grudge against anyone who comes down on the different side of the fence on these ones.
 Very close to calling this depiction ambiguous, but the traditional sources for the dryad were just enough to tip me over to one side. Much the same situation with the lamia.
 It’s always a tricky thing to sex dwarvenkind by their beards, but this default position seems relatively safe.
 There’s so much variety in even the real-world kingdom of fungi that I hestitate to assume much more.
 I don’t think it’s a stretch too far to lump mermaids in with the merfolk, so I have.
 I live in hope that someday we’ll see a female minotaur with battle-ready armoured udder, but let’s say that I’m not holding my breath.
 Every monster could adopt these prefixes. “You’re attacked by three androbugbears and four gynogoblins!” It might clear up a lot of confusion.