"nicer" isn't the definition of a euphemism. Lots of words sound nicer than other words but that doesn't imply the less nice words need covering up.
An actual online definition:
an inoffensive word or phrase substituted for one considered offensive or hurtful, esp one concerned with religion, sex, death, or excreta. Examples of euphemisms are sleep with for have sexual intercourse with; departed for dead; relieve oneself for urinate
As you say, "homosexual" isn't taboo in any way, so it doesn't need euphemisms. Gay is just a synonym, and we are all free to choose synonyms, whether on the basis of niceness or not.