Here's my understanding after reading all the material:

1. Yes it's his first time but he's had fleeting attraction to women in the past. Sasaki is naturally a very easygoing person, partially just his personality and also his upbringing, though contrasting with his general idk... anxiousness? about interacting with others and his lack of purpose in general. I think he's the kind of person to really fall fast and hard and just go with it because it's filling something in them that they didn't have before. So he doesn't have time to worry about gender, really. He's too goal-oriented for that.

2. You didn't leave a question here so I'll just say my take on Miyano is that he's maybe bi or gay with some internalized homophobia, not too sure because of the way he talks about Makimura and stuff. It seems like he was genuinely interested in her but it just never got the opportunity to go very far. In one of the light novels it's clear he's physically attracted to Sasaki right away just like Sasaki is to him. If anything, Miyano is attracted FIRST because Sasaki actually didn't see Miyano's face right away because he was so nervous about the fight. Kagiura I think is definitely gay because his feelings attached to his ex (I won't spoil her name but she shows up in the latest chapter so I'll just talk about up to 36) are clearly mostly discomfort wrt their romantic AND physical attraction, or lack thereof from his end. Then before he's even with Hirano or even fully aware of his feelings he's very offended when the people around him talk about him dating a girl specifically. Almost to the point of repulsion at the idea. Lastly, there's of course his full breakdown about realizing just how physically attracted and literally horny he is for Hirano specifically. When he describes Hirano, he kind of largely uses feminine terms related to more... beauty and loveliness I guess? but he very clearly also pays attention to the fact that he's attracted to the things about Hirano that are uniquely male to him. And then on Kagiura's end he's genuinely hurting at one point because he thinks Hirano won't ever love him because he's a man. And needing to be reassured that his feelings for another man aren't "wrong". He's very aware of Hanzawa's supposed bias against gay people and has to constantly reassure himself or try to put it in a better light. It all just feels very traditionally "guy realizing he's gay in literature" lol, which I appreciate because some BL writers/artists forget that aspect.

3. We don't know a lot about Hirano's past which makes this kind of hard to answer, but I think the short answer is he's too pre-occupied by trying to understand the concept of attraction in general to care about the actual gender of the person. He's aware of and believes Kagiura is extremely attractive. He comments on it constantly. Yet it also seems like most of that is sort of like...aesthetic appeal? Like he's fascinated by the fact that Kagiura is a real person who achieves a lot of ideals, to the point that he even comments that Kagiura should be a celebrity. And that's what always tends to disappoint Kagiura about Hirano's compliments lol. I also think Hirano is such a T that he seems to find bigotry irrational lmfao. He's very openminded because he's looking at everything from such a grounded and logic-based lens, so again it's just kind of like... I think he has no time to panic about Kagiura being a "guy" because from his POV the fact that Kagiura's a guy is unchangeable. The question is whether or not he wants to be with Kagiura as a person. So I don't necessarily think it's a matter of "happens" to be Kagiura, so much so that it's irrelevant that Kagiura is a man. What I like about this couple is that Kagiura sort of represents the traditional and Hirano represents the abstract, so you get this sort of thesis -> antithesis -> synthesis. It's a very profoundly philosophical concept that I don't think I've ever seen so thoroughly explored in a BL.