Song by John Donne

Recently, I read John Donne's Song: go and catch a falling star. Personally, I think Donne launched an attack on women's infidelity which was a commonly believed quality found in women at that time.
but my teacher said that the theme of the poem rests on the last lines of the first stanza, " and find, what wind, serves to advance an honest mind". He said that Donne meant to criticise the social customs or manners of his time.
Then, if so, what is the point of the next to stanzas which focus on women's inconsistancy? Do we really need to find out something serious or morally instructive in every poem? Couldn't one just assume the poem as a mere expression of the poet's thought?
Can anyone give me some advice? Thank you very much.

Firstly I think you need to read the poem in question more closely. The next two stanzas do not focus 'on women's inconsistency'. Re-read it carefully, try to think what Donne is really focussing on, because it is not really 'women's inconsistency'.

Your second question is confused. Think about the diff. between narrator and poet, especially when it comes to Donne. No - finding morality in poetry has nothing to do with reading a poem (though a poem could survey some moral issue).

The poem begins with a catalogue of impossibilities - none of these things in the first stanza can be done.

In the same way, he says that even if you are gifted in discovering mysteries, and although you search until you grow old ("till age snow white hairs on thee"), when you return you can relate all your discoveries but still not have found a single woman who is both true (faithful) and fair (beautiful).

And even if you do manage the impossible, and actually find one who is both faithful and beautiful, it would be no use the speaker going to see her - because, (he jokes) by the time he arrives (even if she lives nextdoor) she will have been unfaithful to two or three other men.

The poem is an elaborate joke, an exaggerated way of saying "there is no such thing as a beautiful, unfaithful woman".