Because this is mostly a real-time action game, you will need compensation if you are punishing the player. You can dodge or reposition if your speed is halved.
Idea: the healing while fighting is greatly reduced and continues slowly while removing the semen in the womb. (can also continue while in sex-mode -> uses the time of the player not continuing the game)
Maybe there are mods or genetic adaptations to have a bigger womb and hold more content. There could be a lot of mechanics you could implement, but
Nayenezgani doesn't have unlimited time. I would like to increase the baby mechanics to maybe cling to the breasts and survive longer. This means you could use a limited count outside of your body, relative to your breast milk production. Then you need time to birth more, but only if there are some in your womb.
If all relevant things have a weight, you can calculate how encumbered the player is. Would also limit heavy weapons and armor or you will need something to increase your carrying capacity. I would prefer a gradual reduction, maybe reducing max stamina or something.
Remember: Maybe there are some people that do not like the idea or the visual of inflation, stomach bulge or pregnancy. I would like to hear their input and ideas.