I also dont get why its dehumanizing. Its descriptive, not prescriptive.
But there's a choice in what characteristics we narrow in on, when describing someone or something. It's not completely objective or value-neutral. saying "homosexual" for gay people puts the focus on their sexual choices as defining who they are. We don't trot out "heterosexual" with equal frequency, except when needed to differentiate people as "not the gays".
A similar thing happens with video games that have characters defined by some single trait. For
white male characters, the defining characteristic is their
job, but for other characters their defining trait might be "is black" "is asian" or "is a woman", with the barest of stereotypes on top of that. The black guy is Muhammad Ali, the asian guy is Bruce Lee, the woman is acrobatic. It's similar to Original D&D where Humans were defined by the class they picked, but Dwarf, Elf and Halfling were "classes", not races. It's only acceptable because they're pretend races. If they were real races in OD&D it would be remembered as basically the most offensive thing in RPG history.
Straight people are described by what they do for a living, whereas "homosexuals" are described by sexuality. This is how it dehumanizes people. It's similar with crime reports. If a straight white male commits a crime, we'd describe him in
some other terms: "an electrician from New Hampshire went on a shooting spree", whereas if a black man commits a crime, then it's reported as a black man did it, and you probably won't be told what his career is at all.