I think it's just your unconscious mind wondering "what if". It doesn't mean you actually want or plan to have a same-sex encounter in waking life.

Since you were both pretending to be the opposite sex, then maybe your unconscious just went the next step and asked, "What if instead of being with someone who is pretending to be a woman, I was really with a woman?"

Your dreams provide you with an opportunity to try things that you would never do in your waking life.

The bridge could represent the path to your unconscious mind (where you are free to experiment with things like lesbianism). The two girls sound like they are the parts of you that are still aware that this isn't right for you, trying to force your waking-life restrictions onto your dream life.