I tend to think that Neruda is talking about the possibility or potential of the man in the poem engendering a child with the unnamed woman and that, were this to occur, this potential child (or the life flowing through the child’s veins) would then, through her/his existence, “ground”him (a “sailor” who, instead, would like to have a woman waiting at every port). Meanwhile he is likely playing here with the words “amarrar” - “to tie up, to bind” - and “amar” - “to love.” Later in the poem he also seems to play with “a amar” and “a mar” as in “to love” and “to the sea,” probably explaining his introduction of the male lover as a sailor.
The line quoted is also full of alliteration and semi-hidden meanings:
Para que nada nos amarre que no nos una nada.
For example the word “nada” means “nothing” and “swims,” as in “una nada” or “a female swims.” Moreover, the word “una” means “joins,” which carries the surface meaning of “nothing join (us)” as well as “a nothing(ness)”.
amarre
is a conjugation of the verb for tying/binding, anduna
isunite
.