Your definition of "partner" is very limited. You might want look at some other definitions before making potentially offensive statements like that:

http://en.wikipedia.org/wiki/Partner

In this case, the mother's partner is another woman, implying that the woman is in a lesbian relationship. So your joke about 'meant to say spouse' could be quite hurtful if they would actually like to be married but can't because of current marriage laws.

Not to mention that your comment simply sounds ignorant and/or homophobic. Just thought I'd clue you in to why you're getting downvoted, in case it wasn't obvious.

If you parse the pronouns carefully, you'll see that both the mother and the partner are female.

There are an awful lot of places where they legally cannot become spouses.

"Partner" is acceptable english usage for "significant other".

There are plenty of occasions when one parent of a child may be with, but not married to, someone who is not the parent of the child.

My boyfriend is my partner, but not my husband, though we've been around for longer than many marriages. For some reason, I don't think Chris would have had a problem saying wife were it so.

I respect the fight, but we don't all fit into the marriage binary either.

(And hey, maybe someday the two of us will be married too, heh.)