How to refute the idea that Buddhism might be actually "too extreme"?
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?