It's just the game's built-in mechanics. You have to trigger a random dialogue first to get the 'sex' option. Once it shows up and you pick it, the game rolls another RNG for pregnancy. There's no pity system for either of these random checks. So, if you're lucky, you might get the option several times in a row, or even hit the jackpot on the second roll on your very first try. But if RNGesus isn't on your side, you could go through dozens of dialogues without triggering it once.