By "intimate" do you mean long-term relationship or simply casual sex? If the latter, this seems like a rationalization. There are only "social complexities" involved with transsexuals because people are idiots; just because so many people are bigots does not mean transsexuals should openly advertise their genetic history. In fact, it's exactly what keeps them in the closet about it. Who can blame them?
I still don't see why it matters. I do understand that it seems weird for people who have not been exposed to it before; it certainly seems weird to me. But, unlike so many people, I actually question that feeling and ask myself if it's justified. And it doesn't take much to realize that it's not.
As I said before, forget typical transsexuals for a bit. Think about this in the context of people born with a genetic condition so that they had ambiguous genitalia, and their parents basically chose a sex for them. Such a woman was not born a woman, and had to have surgery to make her so. Do you think it'd be reasonable to feel "tricked" if she casually slept with you without disclosing that fact? That just seems outrageously bigoted and unwarranted. And I don't see any meaningful distinction between that case and any other transsexuals. The bottom line is it shouldn't matter, but it will continue to matter such a great deal as long as people aren't called out on it.