So, within this context, "com" doesn't mean "with"?
I guessed "I dream with my girlfriend" but I'm assuming that the word functions differently with this verb.
Yeah this is the same in Spanish - they use with, but they mean about. There are some verbs where you just have to memorize the prepositions for certain verbs. :)
To distinguish "I dream about my girlfriend" from "I dream with my girlfriend" you would say "Sonho junto com a minha namorada" for "I dream with".
Hey gringo! ;) Sobre = ok, but not for a person. That could be like an event, or something, but not people.
My native language is Spanish, not Portuguese, but I'll offer my opinion because the languages are similar in so many ways.
In Spanish, we always say "soñé con" (literally "I dreamed with" but in meaning "I dreamed of") when speaking of a person or a character (a pet, a monster, etc.). We will also say "soñé con" when speaking of something that can be stated as a goal. For example, "soñé con ir a España" (I dreamed of going to Spain).
We say "soñé que" ("I dreamed that") when speaking of a situation, e.g., I dreamed that there was a fire, I dreamed that I went to the moon, etc.
Actually "I dream with my girlfriend is also a translation of this sentence so should be correct. It is a joint dream of ours, we dream together.
Because "beloved" could be your wife or your sister, or even your mom. It doesn't only apply to your girlfriend.