well maybe you'd need to take into account the fact fred is now closer to 40yo than 20yo, and as i already said,he's not using viagra in every shoot like some people have claimed.
not to mention, the girls he's actually working with now are quite far from the level of the babes he had back when he was working for JJ.
let be honest, if we were used to bang nymphomaniac victoria's secret models on a daily basis, and then have to come back to everyday average girls, we probably wouldn't have an a raging boner every single time aswell. and i guess it only get worst when you already had sex with so many different women.
another factor could be the "shooting Schedule" (pun intended), maybe he's not giving himself enought time between each scènes, but we don't know if there's not exterior factor forcing it to be that way (actresses agenda, location available for a limited time, ...)
or even simplier, porn actors aren't machines. they're still human, sometimes there's just no chemistry between two people, and even if they're professional you just can't help it.
i'm sorry to pop some of the most naive people dreams, but pornstarlettes are more Efukt than wedding material, i'd be gentle saying most got more issues than times magazine.
if you're just as fuck'd up as they are, i guess it's perfectly fine, but when you're an right minded, well spoken person like him, it must feel pretty weird.
but on the subject of more and more girls being able to handle him, i'd like to point to the fact that, 15 years ago you had to search very hard to find a DPP or a DAP, and interracial wasn't mainstream at all, nowaday it has become common.
what i'm trying to say is, old days pornstars weren't forced to get the kind of stretching they do nowadays. there's no Wonder girls like kagney lynn karter or Arianna jollee are able to take him ball deep, have u ever seen those two in a gang bang ? they'll milk a dozen guys without breaking a sweat, and call it breakfast.