Why is teenage pregnancy frowned upon?

The teenager cannot legally lease an apartment, nor car, nor have they established the financial preparation by saving and investing to comfortably support the child. Being a parent requires responsibility. If one has to place the financial responsibility of supporting their own child on someone else, than they show lack of responsibility. Society does not expect a teen to have graduated college and settled nicely into a good paying career as schooling is not geared toward that being completed in teen years as one would have had to start this preparation in grade school, to which maturity level wise, it would not be possible. 

So, the teen cannot afford the child. Has not acquired life experience that comes with the adult rites of passage such as budgeting, paying ones own bills, planning, and one is expected to leave childhood fun behind to become an adult at a time when society has geared for them to still enjoy life as teens and enjoy high school proms, games, sports, and develop their personality. So, teens have not acquired the life skills necessary to be a good parent and being forced to do this early is detrimental to future success and emotional well being

Because childbirth is dangerous for young teens and then of course there's the fact that if adoption isn't happening the baby's life will be terrible almost all of the time. Most teens aren't Kylie Jenner and therefore most babies born to teens live in generational poverty, do poorly in school and never quite get a decent shot at life. 

Every taxpayer who ends up having to support the ba$tard child probably wishes the breeders would be more responsible and refrain from doing so until they have the resources and maturity to take care of their own spawns.