My view on RP: Bring it! The more RP, the better. RP is something I think people whom the activity interests need when things start to feel dry. It adds a dimension to the game that can't otherwise be attained. RP wicks the imagination and allows for folks to release their creative side in a safe and fun way.
My view on ERP: Okay, so you want to get risque. That's fine. But here's the thing: not everyone wants to get risque. There are just as many non-risque folks as there are risque folks, perhaps more. I don't like getting too risque, because RP is an activity which yields emotional investment, when the stories get too deep, and because ERP contains the atmosphere of something so personal--something we all want in the real world, to varying degrees--it runs the risk of deteriorating friendships and leading people down dangerous paths, sometimes leading to the intervention of law enforcement.
Why it's taboo: It's different. Difference scares people. Because of that, people tend to make fun of it. I know, because I used to be one of those people. I used to laugh at roleplayers back when I played Ultima Online. But when I tried it, I never wanted to stop. In turn, I was laughed at with all the rest of them.
Just as well, it's gotten to the point where RPers have gained a certain level of notoriety. RPers who take roleplay too seriously (once again, guilty as charged, back in my WoW days) have a tendency to look down on those who do not RP. Those who do not RP, in turn, feel like RPers are using "roleplay" as an excuse to troll or grief them, because "their character is mean, not the player."
ERPers tend to be clingy and shameless, and because of that, they contribute to the taboo nature of RP in general.
Did I miss anything? I wrote this while half-asleep. Have fun ♥