I don't think anyone would disagree with you that opiates can produce some of the longest lasting sex sessions a man can have but as addiction continues the desire for sex tends to take a back seat for the majority of addicts. For me personally it's become more of a take it or leave it sorta thing nowadays. I still think about it but nowhere near the amount a "regular" 20 something guy would. Same goes for genital arousal (i.e. erections). An I don't believe "water weight" would necessarily make you appear necessarily out of shape. It might slightly stretch the skin out and reduce the notability of fine lines and wrinkles. Could be wrong on that assumption though. This study says that estrogen and progesterone suppress physiological responses to the administration of opiate peptide,
http://press.endocrine.org/doi/abs/10.1210/endo-126-2-804?journalCode=endo. So perhaps prolonged exposure to exogenous opiates causes the brain to produce higher amounts of estrogen/progesterone to maintain neurological equilibrium resulting in the aforementioned potential side-effects. I'm not a neurologist/endocrinologist so I could be misinterpreting the info. though.