I'll be honest, I had a hard time thinking about how to reply to this. One: I personally HC him as cis, pansexual and demi-romantic. Two: I also HC that he doesn't really care who he dates, cis, trans, non-binary, agender - whoever. It's who you are as an individual person.
BUT I like to challenge myself and try to interpret him differently. It's a good writing exercise if you wanna include more diversity in your writing. So I apologize if my HCs seem a little all over the place. (As trans myself, I'm still struggling to write trans characters properly.)
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The party began to blur into a cacophony of lights and people. Maybe you had a little too much to drink. It was a quiet celebration at Piers' apartment. Marnie was out of town for the night and Piers wanted to celebrate his first full album being published. The two of you have been dating for about a month now - maybe a little over. You don't remember the exact moment you started taking the relationship seriously, it just sort of... Fell naturally? There's a lot you don't really remember in your currently buzzed state - you just remember he started showing up more and more into your life and you were always happy to see him.
"You alright?" Piers asked, leaning around the wall that separated the kitchen and living room. You lifted your head. When did you lay on the couch?
"Mmh... Yeah." You groan, sitting up and ran your hand through your hair. "You don't mind if I crash here tonight, do you?"
"As if you don't practically live here." He chuckled. "Go get cleaned up. I'll meet you in the bedroom." You nod and stumble your way to the bathroom down the hall to the left. You splash some water onto your face and cup your hands to drink some to settle your gurgling stomach. You're not sure if it helped or not.
You then go to the end of the hall, peering into Piers' room, where he's laying on his back, watching you between the strands of his bangs.
"Hello, love." He greets with a tired smile. You grin a little and fall to his side, wrapping his arm around your shoulders to bring you closer. He kisses you once, twice... The gentle warmth of his lips is addicting. Just one more... Just one more...
Well, one thing led to another and the next thing you know, his hands are under your shirt, running down your bare sides. Almost instinctively, you flinch back and press your clothes firmly to your skin. Piers' eyes grow wide.
"Did I do somethin' wrong?" He asks in a soft whisper. "Too fast?"
"I... I'm just...." You murmur. Fuck, that's right. You haven't told him yet... "My body, I mean... It's... I'm...." He lets out a soft sigh through his nose, watching you verbally stumble. He brushes the hair from your face.
"Love. I don't care." He states gently. "Besides, I thought you'd figure it out by now." Your brows furrow. Figured out what? Was there something to figure out. He lets out another soft sigh and lifts his shirt, showing a black binder over his chest. For a moment, you think this is just some sort of dream, but his voice comes back to you,
"If anything, that's how I know I can truly be me with you. I thought you knew that about me by now." He pouted a little and poked your forehead. "Or are you too drunk to think?" He lightly scolds. You feel heat rise to your face. He chuckles and pulls the covers over the both of you. "Hey, don't worry about it tonight. You can fill me in with details later. Just want to let you know, this doesn't change anything." He assures you, pressing a kiss onto your forehead. "Tell you what, let's just sleep it off for now and talk about it later, okay? I still got you..."