people assume it's harry because he's the currently popular one who uses queerness as a marketing tactic. back in the day when 1d were together, literally all of them had gay rumors. like, when louis had his son, there were fans who legit thought the baby was fake, because no way in hell he'd fuck a girl. and the 2 popular ships from that time were larry and ziam. so obviously people also thought not just harry and louis were gay, also liam and zayn.
so based on the rumors and fandom behavior from when 1d was a thing, it could literally be any of them