User blog comment:Zolnir/Volume 2 Episode 1: Best Day Ever Review and Theories/@comment-10390252-20140725112116

I think it's worth noting that, to personalities like Mercury and Emerald, sex is just that - an action carried out for pleasure. It doesn't even have to have an emotional content or even to involve mutual attraction.

If, as you suspect, Emerald and Mercury were raised togehter by Cinder, it's possible that they've been through an awkward puberty together, including some toe-curling hormonal incidents. Mercury is snarky enough that he likes calling back to that when Emerald starts getting uppity with him. He isn't really coming on to her, just reminding her of an embarrassing past momment or two.