Not quite true, IMHO.
When I was taking deca (400mg) + test (500mg) before, I still had problems. I would always wake up with hard-ons (very hard ones from the test), I was horny, and had absolutely no problems getting it up (heck, just touching my girl would get it up) and keeping it up (if there was enough stimulation - visual and physical), but once I paused to put on a condom on or switch position or something... BAM, away it went. And with BAM I literally mean within 10 seconds. Sucked... but it'll go away after the cycle.