You would have to describe it and use it within some context. For example, you could utilize these words: gro/gron (bound/bind), gein (one), ahmul/kiim (husband/wife). For the more "physical" translation for "mate", you can use creative euphamism or utilize slen (flesh) or kopraan (body) for context.

Possible examples (You can mix and match):

"Ney los gro ol ahmul ahrk kiim" (Both are bound as husband and wife)

"Mu los gro ol gein" (We are bound as one)

"(Name) los ahmuli / kiimi" ("Name" is my husband / wife)