You need them at 4 hearts, then bang them until you get the popup.
And “preventing her from getting pregnant” it has many reasons for that.
One of them is the girl being already pregnant.
Another one, that could actually stop you from using the system altogether, is meeting a character that talks you about the pregnancy content.
You need to talk to the council girls in their demesnes through the nexus, so she takes an interest on you.
And if you don’t know what the nexus is, progress with Pixie until you get invited.