But, my good lord, I wot not by what power,--
But by some power it is,--my love to Hermia,
Melted as the snow, seems to me now
As the remembrance of an idle gaud
Which in my childhood I did dote upon;
And all the faith, the virtue of my heart,
The object and the pleasure of mine eye,
Is only Helena. To her, my lord,
Was I betroth'd ere I saw Hermia:
But, like in sickness, did I loathe this food;
But, as in health, come to my natural taste,
Now I do wish it, love it, long for it,
And will for evermore be true to it.

It is the magic that did this.

In the final scene, Titania says the following:
TITANIA
First, rehearse your song by rote
To each word a warbling note:
Hand in hand, with fairy grace,
Will we sing, and bless this place.

That's a clear statement that Oberon and Titania will bless the lovers generally.

Oberon then says the following:
To the best bride-bed will we,
Which by us shall blessed be;
And the issue there create
Ever shall be fortunate.
So shall all the couples three
Ever true in loving be;
And the blots of Nature's hand
Shall not in their issue stand;
Never mole, hare lip, nor scar,
Nor mark prodigious, such as are
Despised in nativity,
Shall upon their children be.

That's a clear statement