Unlike homosexuality, heterosexuality is "immutable". To define heterosexuality as merely sexual conduct between people of compatible genders is to suppress a fundamental truth about what it means to be human. All human beings with the exception of hermaphrodites (people with genital deformities) are born with a reproductive system that is heterosexual by nature. We are either male or female. We have sexual feelings only because of chemical and other processes that are rooted in our procreative heterosexual design. Thus, a male sexual orientation toward a female (or vise versa) is self-evidently normal and natural.
By contrast, a male-to-male or female-to-female orientation is self-evidently abnormal and unnatural. For homosexuality to be equivalent to heterosexuality, it would need to be rooted in its own homosexual physiology.
Which is why it shouldn't be celebrated but cured.