Even if there were tags for smut I doubt most people would put it down straight off. Personally, if there were to be any erotic scenes, I would like a build up to that point - something I feel an RP tagged as smut wouldn't offer - and I expect the majority of female RPers are the same. 

Tagging the RP as smut would draw, as a majority, sexually frustrated boys who are hoping to RP with the fairer sex but instead find themselves in Elberta around festival time*. 

Sure adult content does suggest some of this - rather than spicing the role play up with filling out your council tax forms and mortgage meetings at the bank - but to a lesser extent.

*Elberta holds a huge sausage festival every year.