Thanks for the welcome and the interesting article!

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.

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.