Downers make the brain lazy and you end up going with the mentality of 'any hole's a goal', the beer goggles take hold and the good-looking average girl you went home with the night before turns out to be a heffer. The lowering of inhibitions seems to lower your mental filters and makes you less choosy.
Uppers send you on weird obsessive mental drives. It's hard to describe but it's not even about the sex anymore, it's no longer the 'doing' of sex that feels good but more like something else. I can't put it into words but it turns sex into this disgusting mechanical thing that only serves to feed the dopamine train, rather than an act of love.
I haven't touched meth in about 6 or 7 years and don't ever intend to again. It puts you in a weird headspace that I found incredibly unpleasant (not to mention the eventual comedown and the fact you stink like a hardware store chemical aisle). I will admit that the very first time IV'ing it is one of the single most incredible sensations you can ever experience but everything after that is just chasing the dragon, a waste of time and money that slowly destroys you physically and mentally. It sends you down the pathways that lead to the darkest urges within the psyche and, with enough use, will render a person nothing more than a basal animal chasing a high at the expense of anyone and anything.