Morality (UPB) stands the test of logic. It means it is in the fundamental structure of our Universe and how it functions in reality, whether we like it or not.
You go against the fundamental principles of the universe? fine, you end up doing a lot of damage to everybody and yourself, get sick shortly, and you end up miserable.
(Physics --> clear logic --> math --> morality is the way real world works. Not something that exist "only in your head", no. Only the transient language we are using to describe them are in our head.
Example: if humanity dies today, there will be nobody saying:
2+2=4
Earth and Moon are 93,000,000 miles apart
But it will not change the fact that:
2+2=4
Earth and Moon are 93,000,000 miles apart
If aliens from Moon want to come to Earth they will have to fly 93,000,000 miles, and if they build a pyramid on Earth using sound 2+2=4 math, the pyramid will still stand strong as a product existing in beautiful harmony with fundamental principles this universe works.)
Imagine real life situation. You're on a date or a job interview. If you seriously tell your life and work partners that you rely on 2+1=5 math in your life, who would want to deal with you?
If you build a skyscraper or an airplane based on such science and use them, the result will be your death.
And you will miss opportunities to create real advances human life relies on, because math is needed for it. So you'll have no joy of creative accomplishment.
Morality to life and relationships is like math to advanced creative achievement. To have the happiness and joy of life long term you will need to have meaningful honest relationships, and for that ethics is a must.
Conclusion:
Morality / ethics are voluntary. Just like using math / science too is voluntary.
(Many useful things are voluntary, You are not obligated to use neither human language nor body language to quickly explain to a doctor where is it that you are bleeding.)
It keeps your life incredibly better and more advanced, but it is voluntary.
However, regardless if you choose to use them or not, the fact is that they do exist as fundamental principles this world operates. So you can go trolling around saying "2+1=5", "murder is nothing wrong and not immoral" and seemingly "nothing happens", but you will just be wrong!