Vulcans have done such a good job of controlling feelings and urges that their self-control has somehow become ingrained in their biology. Pon farr came about, by design or naturally, as a way to keep the race going, despite their control and unemotionalism. (Hey, maybe a dormant gene from a previous species reactivated, some animal with a 7 year cycle).
They never say Vulcans can't mate otherwise... though it could be true... maybe it's more that they don't, and therefore that they need forcing.
The line between "can't" and "won't" may be blurry, or nonexistant. If a Vulcan renounces the disciplines and works against them, he probably could go at it at will, like any other species, though it probably take time. Sybok probably has a pretty good time...
