Having pets rather than kids should be considered a psychiatric disorder called "replacement baby syndrome", at least according to G. Shane Morris.
Millennials tend to struggle financially. Raising kids is expensive. Of course, a child's intrinsic value as a human has no price, but food, diapers, educational materials, and other things kids need all come with some form of a monetary cost. The cost of raising a child from birth to age 18, according to a report from the Department of Agriculture, is about $240,000. That's pretty financially prohibitive for someone who's young and caught in the middle of an economic recession. Of course, this cost can be reduced via smart budgeting and cost-cutting wherever possible, but it's not just financial reasons that prevent millennials from having children.
Having children comes with philosophical concerns whether future parents are willing to think about them or not. If these children, who are human beings with their own independent desires and beliefs, do not fulfill the parents' vision, what purpose does their existence serve? These are questions everyone asks themselves at some point in their lives. Deep human existence questions aside, every parent will influence their child's religious and political beliefs. What would you do if they rebel against your influence or worse: if they accept everything you say at face value and never test it? Perhaps millennials do not want to bother themselves with so directly influencing another human being.
Looking down on the younger generation is not a new phenomenon. Older generation "supremacy" has been around since technology and social norms have evolved which is...basically as long as humans have existed. Not only that, these people tend to dislike anything to do with the younger generation unless it serves their interests. (Kyle Kashuv, anyone? That guy is awesome.) Most of the time, the behaviors we see today are not new but are so ingrained in social behavior that we don't even realize that the patterns exist. Seeing as older generation supremacy is generally connected with social conservatism, it makes sense that the decrease of a desire to have kids would come off as a threat to traditions and standards that hold society in place.
There are people who honestly treat their pets like replacement children in ways that can very well be pathologized, but this is far from the case with most sensible pet owners. Some people have a strong nurturing/parenting instinct, but do not trust themselves with the level of influence that a parent would have in a child's life. Others are plagued by mental and/or physical illnesses or have little interest in human children, but love animals. That said, I think it is rather disturbing to hate children simply for being children, which is essentially hating someone for existing. They are people trying to get by in the world just like anyone else.
On another note, you don't have to raise children from birth. You can adopt a child, especially an older child that was overlooked in favor of younger children, if you don't feel comfortable with pregnancy or raising children from birth. A commitment to a child is a commitment to their wellbeing no matter who they turn out to be. If you don't want to support a gay, trans, autistic, physically disabled, or otherwise "flawed" child, don't have children at all. Whether you want pets, kids, both, or neither, weigh the pros and cons against each other, ask the philosophical questions, do research, budget, be honest with yourself, and decide accordingly. Chances are you'll change your mind, but this is not the case with everyone. Pets in place of kids should not be pathologized unless there are other symptoms to suggest that one is suffering from a mental illness, in which case they should not be shamed but helped.
Sources and Photos
http://thefederalist.com/2017/05/09/pets-instead-kids-considered-psychiatric-disorder/
http://time.com/money/3136260/245340-usda-cost-of-raising-a-child-having-baby-245000-304480-department-of-agriculture/
https://quoteinvestigator.com/category/gene-zirkel/
https://www.verywellmind.com/what-is-a-psychological-disorder-2795767
https://www.pinterest.com/pin/446137906807454098/