I remember finding this manga here, but can't seem to remember the name of it...
It starts out with a hetero relationship (girl x boy); then, the boyfriend is revealed to be a trans (male to female). Even with the boyfriend being a trans, the two continue their relationship (now girl x girl). I am pretty sure it has multiple chapters, and the art style is on the realistic side.
Thanks, your help is much appreciated.

I think I've read that one before...do you remember if the girlfriend was being hit on by another guy and the transgender girl sort of confronted them? The guy says something like "Woah! Tall!" And then the girl introduces her as her "girlfriend"? By the end I think the transgender girl says "Maybe I should wait until we have kids to get the sex change..."

I'm sure I've read that one somewhere. If it doesn't sound familiar then I could be wrong.

YES THAT IS EXACTLY IT.
Do you still remember the title? I remember that I really liked the story and wanted to read it again, but then I got stuck in this rut.

The two-part story "Ayumi and Aika" from Mermaid line

No wonder I couldn't find it - it was part of a larger compilation.
You are one of my favorite people in the world right now; thank you so much!