It’s not a switch, not in a meaningful way. It’s the time spent together that forms the bond. I don’t love my kid because I had sex to have him, I do because I taught him how to walks and speak, go to the toilet and play with marbles and had all kinds of fun in the process. I love him because every time I show him something new, or I let him have a spoon of my coffee (he loves it) there’s a wide smile appearing on his face followed by “I love you daddy”. Its the most validating, gratifying experience I had in my life, even if it’s a pain in the ass and I’d rather watch tv or tinker in the garage instead of sitting on the bleachers of this gym while he practices somersaults. If I adopted him it would be largely the same thing.