I'll answer, and please don't pass judgement on my being honest, since you did ask!!

Personally, I think they do
look dirty. I
know that they aren't, but dreads look so rough, unkempt and dull, that that is how they
appear.
I've never had any thoughts regarding pot-smokers or the sex partners of dread-wearers (I don't care about either). In fact, all the pot smokers I've ever known were yuppie types (I've never smoked the stuff, I hate
any kind of smoke :Puke )
Dirty hippie? Well, I was around when the hippies got their start (the first time, back in the 60's!

), and I remember girls with long hair (and, guys, too, naturally), but not many with dreads.
I love the look of long hair (mine is 52", from crown to ends). But, I like it without the look of tangles and snarls, swinging free or braided, looking shiny and healthy.
All that being said, I have never given anyone with dreads any kind of "look" because it isn't of any interest to me. What you want to do with your body is your business!
I don't like tattoos or body piercings (beyond the ears), either!

Just giving my honest opinion, I mean no offense.
