these are not the same. You can't equate them.
obesity is genetic
It is not genetic. It may have a genetic component. However it is not 100% genetic.
lifestyle changes will not bring about a skinny person
Lifestyle changes and diet changes will bring about a much healthier person.
sexual lifestyle is genetic
It is uncertain what causes it. There is some suggestion of a genetic component. However, it is not totally genetic.
However, most would say that this is predetermined at birth and can not be changed.
Comparing something that is predetermined at birth versus something that can be changed or modified is comparing apples and oranges.