Some people will say yes... but I say, not if you play your cards right...

I have a splash page on my own website informing visitors that the forum is intended for adults and that eveyone must read the forum rules. But my google ranks is still pretty good.

As well as mentioning my keywords "Politics" and "Forum" a couple of times on the page I have also included them in a Title tag in my main link to the site. Google doesn't just look for the number of occourances of keywords but also the size of the document. A small document with a few keywords can be considered more relevant than a big documents with lots of keywords.

However, once you have your keywords sorted out the most important thing is to get other sites to link to you, otherwise you will find it quite difficult to get the highest rankings.