Without wishing to sound patronising I do feel incredibly sorry for trans people and all people who are confused or unhappy about their gender and/or their sexuality, especially 'pre-op' where they are always forced to have this conversation every time they embark on a new relationship, and risking rejection at every turn.
Looking completely like a woman, but acting like a man and wishing you were a man, people can accept, even if they don't fully undersand it.
Looking like a woman whilst acting/dressing/displaying the body language of a man (to my eyes at least, but I realise that is a contentious view among lesbians) but still being identifying very strongly as a woman (as in a gay, butch woman) on the whole, people can understand, even if it alienates them and bewilders them.
Looking completely like a man, living as a man, sounding completely like a man but with a surprise vagina is much, much more of a head fuck.
Although I haven't seen a post-op 'created' penis or vagina but I'm going to hazard a guess that a conversation still needs to be had there too. I doubt they are so good that you could get away without telling a partner. And if the relationship gets serious then you would be basically living a massive lie to not mention the first 30 odd years or whatever of your life, so even then you are taking a huge risk that when you tell someone you love, they might freak out and ditch you anyway.
Getting your gender 'changed' solves one problem but must create a whole host of others, especially given that a trans person's sexuality is not always straightforward either. For example they may wish to become a woman, but not wish to sleep with men, or vice versa. I imagine it's extremely difficult to know how/where to start in forming 'normal' relationships but to be honest I imagine it's safest to just stick to bisexual people as they are going to be more open minded and less freaked out generally. It's a lot to ask a truly hetero or truly gay person to deal with.
I hope there is a dating agency somewhere that caters for these people and matches them up with people who don't give a toss - because life must be very hard otherwise.