MC knows that he, as a male hybrid wild cat, can get a human man pregnant. But he is running on the assumption that Taejun is a human and not a hybrid.
Under the idea that Taejun is human, for Taejun to say he wants to be a dad implies to MC that Taejun has changed his mind and eventually wants a wife to have kids with. Since a human would live under the knowledge that men can't procreate together.
Also MC still thinks Taejun doesn't know he is a hybrid because humans don't know hybrids exist.
MC found out he could impregnate a human man at the beginning of the story. (It was big news that a male hybrid in his family had a kid with a human man).
Taejun only just found out he could impregnate anyone regardless of gender thanks to his tiger genes/ tiger God.

So what was mc's plan if he knows conception between two men is typically impossible?? I just assumed he didn't know