I'm not sure of all the details, but Amy got pregnant by a schoolmate of hers named Billy, but Billy left her.
And then her and Ste got together and he had a big enough heart to want to raise Leah as his own by the time she was born. It didn't matter to him that she wasn't his biologically because of how he grew up abused and unwanted and alone. The things he wants most in life are love and family, and he loves Leah more than anything.
It's a different situation with Sinead, clearly, but even though it was an unfortunate accident (or so the show wants us to believe right now), and he wouldn't necessarily ever want to have any more kids, he'd take full responsibility if Sinead were pregnant, and he'd be there for that child.