In my humble opinion they're not. That is, if you're referring to "true" transsexuals, i.e. men who feel they are women in the wrong body. Since mentally they're women and they certainly behave like women, physically look like women (some even better) and have affections towards men then we have to conclude they're heterosexual!
But there again some haven't gone for the complete transition and still have penises and in spite of taking hormones, which reduces male sexual urges, sodomise men. So some are homosexual?
It would appear so.