Sexuality being an important part of anyone's life, whatever way one deals with it and her/his personnal impulses... Well, sexuality is a valid way to define a character. Given that, why not roleplaying it ?
The same way a sex scene in a movie can be either a valid part of the plot, revealing who the characters are, or just cheap, wink-wink nudge-nudge soft porn, ERP can probably be seen as a tool to shape characters and their relationship.
And seriously, even if it's just a tool for the players to masturbate vigorously behind their screen, I don't give a fuck. Their keyboards, not mine.
As far as I'm concerned ERP becomes a problem
1) when it's done publicly
2) when it's not consensual
So, basically, when you don't care about other's sensibilities.
And I don't ERP.