let’s start by this time in OOTP when remus order his furious lover to sit and he does it without even arguing. we never see sirius immediatly obeying ANYONE ELSE in the books. if that isn’t a fuckin’ clue.
and then i’ll like to remind everyone that sirius has trust issues due to his family and it would be damn beautiful to see him trusting his moony enough to allow him control over him. their relationship is all about healing the other one, so that makes perfect sense. we also know that sirius in position of control tends to fuck up ( the prank on snape ).
and then there is remus who is always afraid of hurting his friends and had been alone for his entire childhood. if he can top and allow himself to be dominant, it is because sirius makes him feel human and that’s damn beautiful too. and he also likes to have power over his life since there is this massive furry little problem he has no control on.
so I really think they tried the other way around at first ( also because sirius has more experience ) but one day they tried it this way and it was so much better.
and yes thank you tarran I know gay couples usually switch positions, but this is a fandom m’dear : things don’t work like in the boring not printed universe we unfortunately live in.
Fuck JKR 