Varda&Melkor and Marriages of Valar

[Raynor]

but why did they then marry? To compose a union? Why there weren't any male unions then?

I would say it is due to a matter of complementarity; we clearly have pairs of valar who are on the same area of activity, yet they complete each other through their different traits - most striking in the case of Manwe and Varda:

When Manwe there ascends his throne and looks forth, if Varda is beside him, he sees further than all other eyes, through mist, and through darkness, and over the leagues of the sea. And if Manwe is with her, Varda hears more clearly than all other ears the sound of voices that cry from east to west, from the hills and the valleys, and from the dark places that Melkor has made upon Earth

-while with Aule and Yavanna we could even talk of opposition, considering that their "children" were at odds with each other; Mandos deals with dooms (active part) while Varie with their recording ("passive")
- Irmo deals with dreams while Este with "rest" (dream/sleep "area")
- Tulkas and Nessa are both outstanding "athlets" - in their own ways
- Orome, lord of animals, Vana lady of flowers

Therefore, each such union helps the "associated" to better complete their mission in their area of activity.