There are not many countries with full legal equality.
It's not only about marriage.
In the US the legal situation is going backwards rapidly thanks to the maga fascists.
Hate crimes on the increase as well.
The civil union vs marriage debate is ancient history in most places so won't dignify it.
It's true other groups face hate crimes
Other groups may have ways of expressing themselves such as Black History month.
Other groups don't face what is very often a trauma and crisis with family, friends, co-workers etc. called COMING OUT.
Pride is especially meaningful to people facing difficult coming out experiences.
People are often forced out before its safe for them.
Pride developed directly from the Stonewall riots.
It is now a major International cultural political phenomenon.