I'm sorry, but "Intolerance of intolerance" is irrational.
It's perfectly logical. If you're goal is to maximize tolerance, it makes perfect sense to not only oppose intolerance, but to pass laws and create systems specifically aimed at disempowering those who attempt to spread it.
Tolerance, as a permissive ideal, is inherently a weak stance. There are many types of intolerance that can threaten to destroy the delicate balance it creates.
Oppression is the end result of tolerating intolerance, because oppression is simply intolerance with power. It's important to remember what intolerance is (and maybe the problem here is working from different definitions?):
Intolerance is an
unwillingness to
allow the existence or expression of states and activities contrary to their own beliefs.
This doesn't mean an intolerance for disagreement, because
disagreement is not intolerance - you should tolerate people who merely think that gay folk should not get married, because that position is not intolerant. If they begin to
advocate legislation to make it impossible for people to do that which they disagree with though, anyone reasonably dedicated to the principles of tolerance and open society should be expected to act against them - to be unwilling to allow them such a victory.
It's like this:
If you believe gay people should not marry each other, but you do not seek to limit them from doing so if they disagree, you are a tolerant person.
If you believe that gay people should be prevented from being able to marry each other, you are engaging in intolerance.
For the tolerant position to obtain dominance, we must seek to suppress the ability for the second group to achieve their desires - we
must be intolerant of such behaviour that seeks to expand intolerance.