I could only be with a guy if I'm not a girl... hence why I always used to joke that I was a gay man trapped in a womans body. Then I actually figured out my gender/sexuality issues... which has added way more complication to that statement.
Order of events ;P
As a child - Thought about being wrong gender, came to the conclusion of being a tomboy.
Preteen - Thought about women more, thought that was normal, thought I was actually imagining myself as them instead of wanting them.
(basically in denial or just not realizing)
Teen - Started coming to grips with being bi-sexual, got into yaoi for a while so that confused things more. Greatly enjoyed various straight pairings (they weren't normal ones however like Grissom/Sara lol) geeklove :3
Late teens - Started seriously thinking about sexuality and that I really did prefer women something to 9 times out of 10.
19/20 - Started publically identifying as gay as well as starting to work out gender issues again. Find the term 'androgyne' and is happy for a while. Use the term bisexual since starting to no longer believe in the gender binary system.
Now - Masculine Identified, floating in the area around bi-male, still strongly attracted to women.
Pre-T - I can only imagine being with women. Post-T+top surgery - I could possibly conceive of having a relationship with a guy (preferably trans in some shape or form and otherwise somewhat non-masculine) Ultimately, I'm much more attracted to women than I am to men.
I could not go out with a straight guy because then that would entail they see me as female and I do. not. want.