This is not an in-depth look at this controversial topic, just a dump of comparison photos. It is mainly to demonstrate that Zayn always treated Harry as a person he was dating. (i.e. a girl).
He would go out of his way to flatter him in order to make him smile, he would elevate him in conversation with others, he openly talked about how sexy he was; repeatedly (something he did not do with any other 1D member despite being in love with Liam lol.) And lastly his body language and demeanor around Harry was always tender, attentive, overprotective, possessive, and thirsty af. More so than we’ve ever seen him demonstrate towards Gigi (or Perrie), even in their staged PR photo/video shoots.
Harry was not Zayn’s dude-bro bandmate friend. He was his lover.