This is what started me on my path to accepting my bisexuality. I brought myself to consider a hypothetical scenario.
What if I were to date someone I believed to be female for a very long time, long enough for me to propose marriage. My fiancee confesses at this time that, before I act on it, I should know she's actually male (certainly it'd be very relevant with this gay marriage stuff). Obviously, this is also under the assumption that we never had sex, as this would very likely (but not absolutely) guarantee the discovery.
Looking at all sides of the issue, I would not recognize that there would be deception on some level, so the possibility of breaking it off with him is possible. Totally regardless of that fact, though, it does not change that I was attracted to this man. Assuming everything else aside from genitalia (even breasts can be altered should the man desire it) is otherwise as I prefer, I would have become attracted to a male enough to propose marriage. The knowledge that this person is actually a male and not female does not change how I have always felt about him.
My inevitable conclusion to this, which I know others will not share, was that the person matters more than the genitals. Anything beyond that would then depend on the individual situation, since as I mentioned previously, there would be some level of dishonesty to consider. Even on the sexual conduct, there are still two holes to work with that are certainly acceptable. In the end, it all comes down to everyone's comfort.
Now on transgenders, I certainly have no problem with that. There's only one problem with it, which would definitely impact a lot of people's decisions. Transgenders are infertile. They cannot create offspring. We've all heard about women who will go out to find men just so they can have children, but the same exists of men, too. There's lots of men out there who are more interested in having children than they are in their potential mate, so infertility would be an outright deal-breaker.
