Seme is a tall blonde with long hair. Really intelligent and researching his "condition" in a lab? Maybe? He's immortal/doesn't age like a normal human being.

The uke is shorter, with short dark hair and is just a normal guy. A salary man?

The Uke finds out the Seme isn't "normal/human" There's a lot of drama.

I can't remember much else except at the end, a friend of theirs sees them many years later and the Seme hasn't changed but the Uke is older...

Oh. It was one volume and it was complete, or at least the main story was.

Please and thank you!

Edit: Oop. Nevermind, found it. Ikusen no Toki o Koete