Why are Millamant's demands not considered to be unreasonable?

In the opening scene when Mirabell discusses the character of his lover Millamant with his friend Fainall he remarks:

"I like her with all her faults; nay, like her for her faults. Her follies are so natural, or so artful, that they become her, and those affectations which in another woman would be odious serve but to make her more agreeable. I'll tell thee, Fainall, she once used me with that insolence that in revenge I took her to pieces, sifted her, and separated her failings: I studied 'em and got 'em by rote.
The catalogue was so large that I was not without hopes, one day or other, to hate her heartily. To which end I so used myself  to think of 'em, that at length, contrary to my design and expectation, they gave me every hour less and less disturbance, till in a few days it became habitual to me to remember  'em without being displeased. They are now grown as familiar to me as my own frailties, and in all probability in a little time longer I shall like 'em as well."

This statement of Mirabell prepares us for the unreasonable conditions or 'provisos'  which Millamant insists on before she agrees to get married to Mirabell in Act IV.

Secondly,Millamant's tone when she states her conditions is playful and it is straightaway obvious that she is not being serious in what she demands. This becomes obvious when Mirabell for his part also playfully lays down his conditions soon after she has completed stating her conditions.

Most importantly, it would have been plain to the audience that the main purpose of the scene was to good humouredly  satirise the power struggle between the husband and wife which is a permanent feature of all marriages and so they would have readily accepted the inflated demands and the exaggerated tone of Millamant.

This scene is one of the most important and finest scenes in all world drama not only for its verbal wit and comedy, but also for the fact that it deals with a universal human condition: this sort of a power struggle exisits in all marriages in all places and cultures in every home all over the world.