Critical Ecstasy is a specific term used in the game, describing the heavily weakened state of a man when forced to ejaculate by a monster. Whenever he orgasms, although he feels pleasured, he suddenly becomes frail and loses the ability to fight back against the monster.
The term is stated by a Knight within Ilias Village, describing that ejaculation drains one's fighting spirit, according to scholars. In other words, if a man comes, his mind and spirit surrenders, and the monster will be able to have their way with him.
No matter the difference between the level of man and monster, the man will surrender if Critical Ecstasy is reached. For example, a level 3 Imp can have her way with a level 59 Luka if she can make him come.
In Monster Girl Quest: Paradox RPG, further elaborations indicate that Monster Girls can also fall prey of Critical Ecstasy and will make them unable to fight for a while. It also affects their pride as Monster Girls, of course. However, there is no explanation on why some Monsters such as the Vampire Girl and Spider Princess can orgasm during their rape scenes, but still remain in control of the rape with no signs of weakening.