What does my dream mean?

It means that you ...

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(wait for it)

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had a dream.

Seriously. Dreams don't mean much. They can be manifestations of inner desires just as readily as they can be long-term memories mixed up with recent events. They are really only useful in that they often get people to talk about their deep and inner feelings. So if this dream is a good conduit to that, then great!

As for your sexuality, I think some of your not wanting to be tied down to a label is a bit related to your graduation. We push people to make decisions. Big, honkin' life decisions. And how the hell can you possibly know, at age 18, what will make you happy for the next 60 or more years? Plus the world will change, a thousand times over. Know what people were doing 60 years ago, in 1950? They listened to records and radio. There were a few lucky souls who had black and white television sets. Cameras were big, clunky things. Films were seen in a theater. Phones were heavy monstrosities, attached to walls or outdoor booths.

See what I mean? The world has changed in 60 years, and it will again, and so will you.

Stop pressuring yourself to make life decisions now, whether they are for your career or your sexuality.

Having same sex attraction does not define your sexuality.
As Jes said, it takes a lot more than that.

Oh good, something real and solid to define.

Freud said, 'sometimes a cigar is just a cigar.' And dreams don't mean anything. It's like asking what every random thought that pops into our head means. Usually it doesn't mean anything. We receive a plethora of information but can only specificly focus on a tiny fraction of it. All the rest gets stored in our subconscious mind. When we dream, this 'full catalog' of info is the source material. How I think of it anyway.

I hate doing dream analysis, but if ya held a gun to my head, maybe your's is you'd like to go to prom with a girl just for the shock value and provoking your Mom. That she figures in your dream but you don't describe her as really homophobic, then she's there for some other reason. And the same-sex aspect may be entirely redundant or symbolic. As in the dream was about other things that what appeared as face value. Some other thing going on, or concern you have about the near-future is manifesting as prom and who you'll go with. Perhaps you're stressing about college or work and leaving your friends, and somehow your Mom is involved with that, so to respond to her you poke and provoke her as with a same-sex date to prom. Examples include she'd pay for school but only the one where you leave you friends, if you go to the one where you keep your friends she wont pay so the metaphor of prom is how that plays out.