Yes, the adult market is a strange one. You'd imagine it to be ultra-competitive. I once remember reading that up to 75% of internet traffic was porn. I'm not sure if this is the case, but it seems that the market has gone way past 'competitive' and way into 'saturated', meaning click prices are decidedly low. Added to this, Google wishes to be a 'family' company and doesn't want to be seen to have particularly great CPCs for adult terms - though I've seen some very high ones on the traffic estimator for Adwords which don't seem to be reflected in what we see from them...
Ed