I don't see how this makes any sense. I could say the same thing about parents having hobbies, friends, a job, etc. For goodness sakes, don't take up bowling! If you like it too much it will divert time and energy from your offspring. Plus the article completely misses the point that a loving relationship with another adult means another adult to help raise your kid.And at least when children are involved, we think that this promise (sexual fidelity) may be morally justified, since extramarital sex can lead to extramarital love that would divert time and energy directly away from existing offspring.
To me the point of the section you quoted is that moderation as key, both for extramarital love and hobbies and everything that you've listed. While I agree with you on your second point, I would imagine that it does take time to reach that point, and that because of that things with your children could suffer in the "inbetween" time. Also, welcome to Hubski!