Alright, here's the deal.
A pokemonXhuman relationship can EXIST if:
1) The pokemon is intelligent enough to understand the relationship,
2) The human possesses genuine feelings for the pokemon, and the pokemon, in return, reciprocates those feelings,
3) The pokemon is capable of consenting to every phase of the relationship.
That's what's needed for the relationship to EXIST. Whether it actually works or not is not major; if both parties are willing to try to maintain the relationship, then said relationship, along with all of the problems it brings, is entirely their concern. This includes things like communication.
And, of course, there are some species who are simply so physically incompatible that any effort at a relationship would have to be kept platonic at best.