
I think it's more of a height type of matter....I've read some Doujins where Haru is seme and Makoto is Uke
Makoto is taller than Haru. Rei is taller than Nagisa (although Nagisa is super seme @///@) Sousuke to rin....but for Haru and Rin, I think they both can be top/bottom....?
I hope this helps you understand some of the fandom's ships.... if I confused you i'm super sorry!!

I get annoyed with seeing the same positions, with 2 people being taken but that's cuz I like double penetration. Also, I really want Rin to be uke, but that's just me. As long as Haru tops.
So hawt!!! But question - how come in every Free! yaoi the sex positions are the same for each character? ( like Haru is an uke, Makoto - seme ect.) Is there a definate way to determine which one is an uke and which one -a seme. I mean, I get that semes are usually more masculine and ukes more feminen, but this is a tough series where all are basicly the same kind of dude, except their personalities differ.