Dabi accidentally got you pregnant.

Even a villain needed something to keep them entertained, and for him, that happened to be a little temporary girlfriend. Let's be honest, who doesn't love a good hook-up with a nice body? The arrangement was mutually beneficial, and it was, for the most part, understood that the relationship was purely for pleasure and entertainment. Maybe. He was gone most of the time anyways, and you had to take on more and more shifts at your job, so surely you both understood that this was temporary. Right?

Unfortunately, he got a bit careless and, well, you got pregnant. He didn't want a child, and he was half tempted to just dump you and be done with the whole issue. It would have been easy to do, considering that he "didn't" gave feelings for you and "understood" that this was only temporary. Yeah, he asked you out, and totally, he said (to himself) that this would end the moment he didn't need her.

But he couldn't. He may have gotten a bit more attached than previously planned. Now, he has to help you figure something out, because despite the feelings he "definitely doesn't have" for you, he can't do a kid. Not at all. Never.