It might be easier to trigger if she has lower Anal XP. To my knowledge, this is the only way to get the slug mark. It is not a status effect or skill, it is a hidden modifier, but you are looking for this line in the battle log. Also important, to trigger the slug scene in the bed she cannot have a slug inside her already.
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Human pregnancy affects the ending, but you can't give birth to a human in game. Monsters have a quick gestation time, but humans take 9 months! (in case you miss that this is a joke, you can't sleep 270 times to advance the human pregnancy).