We can pretty much assume that Hiro is bisexual. In the beginning of the series, Hiro was in love with Shuichi. This is strongly hinted when Shuichi talks about Yuki and Hiro said he was getting lonely now that he doesn't have Shuichi all to himself. I suppose the love gradually faded when Shuichi fell for Yuki and Ayaka appeared. But their friendship love is still strong and Hiro constantly advises Shuichi to sort things out with Yuki. He just want Shuichi to be happy, therefore he didn't try to win Shuichi's heart after Shuichi fell in love with Yuki.

A while after Shuichi fell for Yuki, Usami Ayaka appeared looking for Yuki. From the start, Hiro was probably attracted to her. After he and Shuichi rescued her from some men, he offered to take her to a safe hotel. But we found out the next day that they had gone to an all night karaoke club.^_^ Later, Hiro confesses his love for Ayaka to Shuichi. Ayaka then goes to him and tells him that if Bad Luck sold 1 million records, she will date him. After Bad Luck sold 1 million, they started talking on the phone constantly, up to 10 hours straight. Ayaka lives in Kyoto, so it's pretty far away from Hiro.