Adding a bunch of children that interact with main characters would be way too much effort. Nobody wants to take 3-4 months to come out with updates involving your offspring. You said it yourself: this isn't The Sims, it's an h-game, and people want more h-content without a bunch of complications like watching everyone around you raise your bastard children while you wonder if you should feel guilty or not about popping out more babies than Genghis Khan. The alternative to fast pregnancies is very long or even realistically long pregnancies, both of which are completely unnecessary. Some people want to see the pregnancy content and move on. Imagine if getting Diane pregnant locked you out of all her scenes for 8 months. That would be insane. Pregnancy fetish is great, but I for one don't want to end up playing father figure to 20 different mini-mes or waiting around forever because I got the whole bloody town knocked up.
I don't believe that's what they're saying or what anybody is saying. We're not looking for a SIMS-type game of raising many different children, that would be insane and a complete departure from the genre of this game, we're simply looking for a little more acknowledgement that these kids exist. Keep them as babies, we don't need to see them grow up, but it would be nice if the MC and Diane at least acted like they have a child. Right now, you get about 2 weeks of Diane holding the baby but then it's gone without even so much as a short cutscene to explain where they are and there isn't even any dialogue between the MC and Diane about the baby. Simple dialogue prompts like, "How's the baby doing?", or "How are you doing?", or "I still can't believe I'm a Dad", could be all that's necessary to at least acknowledge the baby's existence. Also, Diane could greet the MC like, "There's my handsome baby-daddy", or something like that.
I don't think this is really asking for much.
P.S. I also think that pregnancy should be a one-and-done feature where, after the child is born, Diane and all future characters who get this feature will automatically go on birth control and will tell the MC during their first interaction after the 2 week period where the baby is no longer seen. Also, I think this feature should be limited to certain characters and used as a proper plot device for them to advance their story and add more character depth. For example, having a child could make Jenny a better person and a teenage pregnancy for Roxxy fits her character, but I don't really think we need to see Mrs Johnson, Mia, Becca, Missy, Miss Bissette, Miss Ross, and all the characters we can fuck and have yet to fuck get pregnant too. If every single female character can get pregnant, it certainly acts as a warning to people playing about unprotected with multiple partners, but it diminishes this feature as a plot device for developing a character.
P.P.S. I also think that it shouldn't be a given that the women will carry their pregnancy to term and that, especially for characters like Jenny and Roxxy, abortion should be an option.