I ran out of fitting Waifus for the Primarchs, so I reached into the warp and somehow pulled out someone from Gotham.
Poison Ivy and Mortarion: one brings beauty and poison, the other brings rot and despair. Together they are the garden and the grave, a match made in the warp.
If you come up with or know of a popular shipping for the outer Primarchs, please let me know.