It's 'cause anything that advertises as erotica very often use both hearts and the colour red, whereas eRP is part rather than central to Iwaku, I was lead to believe. The association is there because it's a common design choice, but not necessarily representative of the full scope of Iwaku. Regardless it's still one of the first thing people will see about their profile when they post as well as about others. Because red is in general associated with passion (and the heart then cements sex) and is one of the first colours to draw the human eye... When I was asked to incorporate hearts in a logo and design that according to the owner had to be red, but then you start testing for reactions and well... That.
Changing the colour would be the easiest hotfix there is if you want to preserve the idea that it's part rather than central. If not, well, I guess you'll see less "but it's so small I didn't see it" and more "well gosh I was paying attention to something else." from the same people ;p