If i get i right (it's hard to mistake, but anyway) - PC need to be imregnated with eggs for having an option to put eggs into the others? But my char wasn't been impreg with eggs when option was available.
The only thing I have seen cause a situation similar to what you describe is when the PC/NPC has the 'Risk of Pregnancy', but the chance is so high that impregnation is guaranteed, the egg laying option would sometimes appear for me even though my character (or the NPC) would still be able to lay eggs even though they weren't technically pregnant yet. The 'Risk of pregnancy' status effect would disappear and it would look like your character (or the NPC) had laid eggs without being pregnant or even at risk of pregnancy.
I may be completely wrong about how the oviposition stuff works, but I am pretty sure that the PC (or NPC) needs to have an egg laying vagina, the character needs to have an ovipositor of some type, and the character needs to be pregnant.
If you met all of those conditions you should be able to lay your eggs inside another character, the exception to that is if that particular orifice already has eggs inside it. So, if you are trying to lay eggs inside a characters ass and they already have eggs previously laid in their ass, you can't lay another set of eggs in them.
Without seeing your character or actually being there to see everything that was going on when you clicked on the lay egg option while your character WAS NOT PREGNANT, I have no idea what you did or what set of circumstances conspired to create the condition that allowed you to lay non-existant eggs inside another character.