Are Catholics obligated to get married with their partner (i.e., Is the Catholic faith against domestic partnerships)?
My understanding is that it depends on what kind of actual relationship exists between the "partners", that they're trying to seal with a domestic partnership.
If these are two people in a romantic relationship, and if they're planning to have sex, then the teaching is pretty squarely that they should get married first. Really married, as in the Sacrament, with all its special graces and built-in pastoral guidance; a secular judge signing a paper can't administer that.
If they're not planning to have sex - if they're not interested in each other that way - but are contemplating a civil domestic partnership for other reasons (tax purposes maybe?) then I think the main problem would be scandal-related. In other words, if a third party someone looks at these "partners", and hears that they're in a domestic partnership, will the third party assume that they're having sex out of wedlock? Do you end up setting an example that "normalizes" sex out of wedlock, even if you're not doing it?
(This is, from what I understand, the same issue a lot of Catholics have with romantic partners "moving in together"; even if you're both staying chaste, what will everyone else assume?)