Rather the latter, but it has nothing to do with the subject at hand – we simply hate using plural pronouns in front of a singular nouns. ;) This even affects the verb choice, for example for a list of singular items:

• „W moim domu jest(!) łazienka, toaleta, kuchnia i trzy pokoje.”
  but at the same time:
• „W moim domu są trzy pokoje, łazienka, toaleta i kuchnia.”

Just an interesting quirk of Polish grammar. As for the matter at hand, you could easily say:

– Moja dziewczyna wzięła mojego psa na spacer, poszli na dwie godziny.

Using „poszły”(not-masculine-personal) would be a grammatical error here, though some Poles would do that if the „pies” was in fact a „suka”("❤❤❤❤❤", as in, a female dog). Still, even then if would be an error, as „pies” is masculine-animate regardless of his/its biological gender, so coupled with a feminine-personal noun can only be used with masculine-personal inflections for a group. ;)

Nevertheless, I think it's not really something worth paying attention to – even some Poles get it wrong and most of the situations are easily covered by the simplification of "Use masculine-personal for a group of masculine-personal nouns, not-masculine-personal otherwise", so unless you want achieve some perfect C2, there is really no point, IMHO. ;)