Pregnancy test wouldn't tell you what is blocking the pregnancy either. That is, if you go realistic route with it
Why the hell is it even blocked? Why do I have to unlock it? I think it's kinda weird, but I don't know enough about gamedev to dispute it
Basically it boils down to story and gameplay having to come together.
This game is story driven. And the way certain characters and their stories go if you had children with said characters too early in the game it would completely break the progression of their story. Take Kana for example. Play her story to its end and you'll immediately see how her being pregnant in certain scenes along that path would just completely ruin the narrative. I won't give you any spoilers because the story is too touching to spoil.
From a gameplay perspective this means he has to block you from knocking her up before a certain point. The question is only how to present this to the player.
He can do it silently and tell you nothing. That works better from a story perspective as you don't get an annoying popup to push you out of immersion. But from a gameplay perspective it's terrible because people will assume its just RNG and they have to grind until they make it. And since they literally can't make it because it's disabled they will get frustrated, than angry, than super angry and here to shout at the dev.
The only alternative is to give you a popup or some other in game indication to tell you "No, not yet. You have to progress the story some more." That might be a bit jarring to some of us but it does save us all a lot of frustration. And it has to be super obvious too. He can't just hide it in a side panel or one of those cards or something because people won't read those until after the point where they are already frustrated and fishing for clues.