Blaise Zabini was bi. He had actually been under the impression that everyone was bisexual when he had first started to consider his preferences during third year. It just didn't make sense to him why people differentiated between genders. Both could be physically attractive and exploring each partner's body and learning how to please each in turn was an enjoyable challenge to him. It was only his observant nature that allowed him to determine that not everyone felt the same as he. During third and fourth years he watched his peers closely and noticed that not many focused their attentions on the same gender.
For example, while he stared at everyone's asses Draco only stared at Granger's. He chuckled to himself and watched his friend tear his eyes away from her long enough to give Blaise a look and ask, "What are you sniggering at?"
Blaise gave Draco a smile and easily made up an excuse. "Weasley's just tried copying Granger's notes again."
Draco looked back over and sure enough Hermione was pink in the face and giving Ron a barely restrained and whispered lecture about how he needed to take his studies more seriously and stop relying on her whenever he wanted to take a nap during lecture. Blaise glanced past the two at Potter and was surprised to meet a pair of green eyes. Harry looked down quickly and continued doodling on his parchment. Interesting, Blaise mused to himself. Had Potter been staring blankly into space or at him directly? It was hard to tell for sure especially during History of Magic, when almost everyone was staring blankly into space. He had always assumed Potter was straight as a rail, but was he? Blaise had never heard any definitive proof one way or the other. He would have to do some digging. Although he was bi, Blaise had some pretty specific tastes. He didn't necessarily have a "type", but he was attracted to the unique and the unusual and Potter was definitely that.
When the bell rang, Hermione stormed out of the room, her skirt swishing behind her. Draco watched her intently until the last glimpse of her shoe was around the door frame as if his eyes were magnetized to her presence. Blaise sighed; Draco was lucky that Gryffindors were so oblivious to nonverbal cues and communication. It took all the subtlety of a freight train before one would notice anything was amiss. However Slytherins were just the opposite; they saw everything and used every action of another person to gather information for their own power. Sometimes assuming things and coming to false conclusions in an attempt to gain control over another. But he would rather be too observant than too dense, Blaise thought to himself as he followed Draco out of the room. He would have to have a talk with his friend about Granger, before anyone else noticed his new obsession with the bookworm.