So, for the demo human pregnancy is a bit more complicated than it will be in the full game.
You should have gotten a quest to unlock prostitution upon completing the forest dungeon at night. That should direct you to talk to Eillana, which unlocks the gallery first, then gives some dialog options.
As the quest says, talk to her about the Hooligans, and then invite her out for drinks to talk about prostitution.
This unlocks the first prostitution client, a hooligan in the upper part of town, signified by a heart balloon icon above his head that pops up repeatedly.
This gives a short tutorial into the prostitution minigame, which then allows you to repeatedly play with him.
From here:
Human pregnancy is slightly different from slime pregnancy, in that it relies heavily on the "Pregnancy Chance" listed now in the body stats screen. This gives you a 5% rate during an actual creampie, but gives additional rolls each night depending on how much semen is inside.
Each night, if you have >1 load of semen inside, it reduces itself by one and rolls for pregnancy. Having more inside at once increases these rolls up to roughly a 25% chance! Each night to get pregnant.
During the full game, you'll be able to play with hooligans right outside of town during combat much like slimes, so this is easy there, but without that, you're going to have to play the minigame with the intent to get more than one load for now.
Leveling up your prostitution level to three gives you access to "Tightening", a skill which increases male orgasm value, and "Teasing", which switches to a handjob and reduces your HP loss, but also reduces their satisfaction (which you need to win the minigame)
This is actually a good thing, as the game is over when he cums.
To avoid running out of HP yourself, using "Deep Breath" the starting active skill, will restore you hp at the cost of your mp.
To restore your mp, you're going to need to complete the quest you get at LV3 prostitution, where Eillana will teach you how to use Mana Drain, a passive skill that restores your mp each time he cums.
From here, you simply play the minigame with the intent to draw it out, switching to teasing stance when the satisfaction gets too high, and swaping to neutral stance for the creampie.
It's kind of convoluted, but it's all optional. You can just spam tightening or even just survive and, as long as you have the HP, you'll get 2 5% rolls each time to get pregnant with humans each day, as the prostitution client also resets when you sleep.
But much like in real life, more is better, so if you want to get pregnant faster, you gotta play around more!