I’ve definitely heard “het’ro”, though this word is not exactly said with great frequency, not in the circles I move in anyway. It’s like how people pronounce “veterinarian”: most of the time it comes out “vet’rinarian”, or “veteran” (as in referring to “Veteran’s Day” for 11/11 in the USA) coming out “Vetrin”.
In the conversations I’ve had that turned around sexual orientation, the term invariably quickly gets shortened to “het” (when used as an alternative to “straight”), to go along with the other one-syllable terms “gay” (for male/male), “lez” (for female/female) and “bi”.