Remus may be a late bloomer physically but he's world more mature than Sirius in some ways. I think that Sirius probably has a girlfriend more to protect Remus because in his mind Remus wasn't interested in that and Sirius having a girlfriend means that Remus doesn't have to worry and Sirius can continue to take care of him without Remus being uncomfortable thinking that Sirius wants more (though he totally does).

It's all very complicated. ahaha Anyway! I agree with your assessment of "thinker and doer."