That's not necessarily a blatant lie. For some people, it does have that effect. Particularly Indicas, which are much more sedative-like than Sativa-dominant strains. Just because it doesn't happen to you personally doesn't mean it doesn't happen to other people who report on the effects of cannabis in studies and surveys. Read about cannabis at the site below. It has all sort of effects, and anything that alters people's neurotransmitters and/or consciousness can, in some people, diminish sexual pleasure.

This is why I like the Erowid site so much. It's scientific, and it shows clearly how different substances have different effects on different people, some positive, some neutral and some negative. You gotta keep an open mind about the bad as well as the good!
Erowid