This challenges the whole notion, "Oh, it different when they are your kids!!" No, it's not. It's harder because society, especially German society, assumes that every woman wants to stay home full time to raise their babies and provides little or no day care or support for women raising infants and toddlers who may want to work part-time or full time after giving birth. This the hard choice: Kids or Career?
Apparently 1 in 5 German men also regret parenthood and the reasons for the regret are often shared by both partners. The study authors are going back to the male respondents to drill down further on the males' regret but my guess is that they cite the change in the relationship dynamics. As we say in the south, "When Momma ain't happy, ain't nobody happy!"