This works as intended, the effect has already been reduced, and I will try to balance it better but the principle will remain the same. There are several factors to it, the MC's sex xp, the girl's sex xp (for the corresponding action) and the girl's stamina, which can all be trained. The girl's max stamina stat will increase (quite rapidly) with every sex action, so you will have much greater chances the more you try. And if you really want to impregnate a total newbie on first try, while yourself being super experienced, the Eruptus spell will help you.
Like I said I will try to balance it some more. It is supposed to represent the fact that an inexperienced girl won't give you much pleasure. And it's not "ejaculation" per se, since you do eventually cum, but not exactly as you wanted to
There is a fixed pleasure gain for every action regardless of your xp or hers, I might just increase that value a bit.