Your example does an exemplery job of explaining your viewpoint on Larry's situation. To explain the presumed viewpoint of Larry's parents on his situation requires merely a very small change; replacing all occurrances of the number 11 with the number 9.

The people who think homosexuality is immoral are objectively mistaken about what is and isn't moral, in the same way the 13-Pebble Favorers are objectively mistaken about the primality of the number 11.

How do you define objective morality? I've heard of several possible definitions, most of which conflict with each other, so I'm a little curious as to which one you've selected.