How to refute the idea that Buddhism might be actually "too extreme"? I actually had a lot of respect for Buddhism, and studied it a few years ago.

But it made me feel that it might be too extreme (and it could be that most religions are too extreme, because they tend to think what they believe is right, and the other beliefs are wrong).  I say that because, for example, it tend to teach self-sacrifice and letting go, and that do not hold onto anything, and to the point that do not marry and do not have any children -- what the deep believers -- monks -- do.

Let's say if this is the appropriate thing to do, and everybody in the world does that, then the world, in 100 years or 200 years, will not have human beings any more (because if no marriage and no children, then there will be no new life on earth, while the existing people pass away).  I can't say that it will be "good" if on earth, there is no more human beings, but lions, wolves, foxes, rabbits, and other animals remain on earth.

Could somebody present a calm and rational argument to suggest otherwise?  I am open to any idea, as I am, like many of us, always learning new things along our life time.
It's kinda funny to see how we westerners stereotypically misunderstand Dharma as postulating some kind of globally applicable set of absolute unconditional prescriptions. Is this not taking it to the extreme? :) Buddha's teaching should not be taken out of context and unwittingly extrapolated.

Did Buddha say lay people should "not marry and do not have any children"? No, he said they should avoid sexual misconduct.

Did Buddha say all lay people should become monks? No, in fact he suggested that monks should not work and are instead to live in dependence on lay community (begging, accepting donations).

Did Buddha teach "self-sacrifice"? No, he taught middle-way: by looking after ourselves we look after others, by looking after others we look after ourselves.

Did Buddha teach radical ascetic rejection of everything? No, what he taught was letting go of obsessions, preconceptions, biases, overgeneralizations, irrational expectations; he taught not to assign too much importance to petty stuff that does not deserve it; he taught to be analytical; he taught to be practical and flexible and wise.

Buddha said, tanha (thirst, craving) is the source of dukkha (troubled mind, emotional suffering). This means, whenever you crave for things to be different than they are, right now, you suffer. This does not mean you should not participate in life, this means if we fully accept what we have, and work with it, instead of wishing it were otherwise, we will not generate emotional suffering.

Is this not the most rational doctrine ever?