your post question drew me to read it. Now that I am here responding to it ( I am a straight male) I think you are leaning strongly on the curious side, if not gay. the dreams you have of being penetrated anally is the red flag. (not that there is anything wrong with being gay...)
I don't have those dreams, never had any dream about having a man take me from behind, and I don't think most heterosexual males have either...
Do you do other things like:
look down to see schlongs at public bathroom urinals?
spend too much time looking at good looking men?
flirting with men?
not sure if those questions are legit qualifiable questions but it sure is fun to ask...lol.
short answer, yes I think you are gay
"Ah, women. They make the highs higher and the lows more frequent." Nietzsche
There are two dilemmas... that rattle the human skull. How do you hold onto someone who won't stay? And how do you get rid of someone who won't go?