I think it's a matter of dosage. As you increase dosage you first start to lose the ability to orgasm then you start to lose the ability to get an erection. I've never lost the ability to get an erection on DXM, but then again I'm usually not attempting to get hard during a dissociative trip (low dose MXE is very doable, though). I've never worked so hard to friggin orgasm as the day after DXM use while on amphetamines. I must've looked like a sweaty sex crazed chimp, but damn what a feeling of triumph and satisfaction when I finally defeated my dick. If I was an older man I'd probably have been risking a heart attack or stoke, heh.