Morality is really a tricky issue. It comes down to this and unless somebody has a different idea, I don't see any other choices:
God: For those who are religious, we get our objective morality from from God as our religious tradition understands it. There are many issues with this not the least of which is what to do when someone doesn't believe in your deity, someone has a different understanding of what the deity requires or someone whose deity has a wholly different view of what is or is not moral in a given situation. The idea here is that there is something or someone that we humans have to answer to. The advantage to religion, at least western religion, is that it values human life over other life. So protecting human life becomes paramount.
Personal Opinion: This is very popular today. Benjamin has a view of morality that others disagree with. Each one of us has their own personal view. To him this is clearly immoral. What makes his view of morality superior to someone whose views differ? Some people would prefer to build hospitals. Others, gas chambers. While the theistic view has it's issues, this is where the atheistic view breaks down. If there is no God, no one to answer to, each person is his/own God and only answers to himself or herself. It's totally subjective. And every view therefore is of equal value and force.
Societal Agreement: When society comes together and by consensus decides something is or is not immoral. The trouble with this is different societies can have different views of what is and is not immoral. In a given society, what is immoral one day can be moral a different day. It usually takes longer than a day, but lets face it American Society has gone through massive moral changes in the last 60-70 years. For example I would wager everyone on this forum considers clitoridectomy to be immoral. Other societies don't. What do we do? Force other societies to follow our "morals" by the force of our arms? What if someone has the military power to force their view of morality on us?
So to throw out the "It's Immoral" argument is troubling because you don't have authority to make it. The US Constitution only mentions life and liberty in Article V in reference to the Government being able to take your life or liberty involving criminal activity and again in Article XIV when it prohibits state governments in the same way. Life, Liberty and the pursuit of Happiness along with references to God given rights appear in the Declaration of Independence and are absent from the Preamble and the original Articles of the Constitution.
It is not an easy topic. This is also a reflection of the state of our educational system. We don't get a classical liberal arts education anymore. We don't as a general rule graduate from college with a "World View". We have a great deal of knowledge but very little wisdom.