Didn't notice this earlier, but I figure I may as well toss in my $0.02.
First off, I'm a straight male, pretty much as straight as they come. I also tend to be very shy and antisocial when it comes to people I don't know (at least in real life, I'm far more comfortable with text), and mostly because of that I've yet to be in any real relationship.
Now, would I want to be in a relationship with someone that used to be a guy? Assuming I'm talking about a full, living together, sex-filled relationship, not just platonic friendship, I'd have to say no. However, the reason is only because I don't believe science has advanced enough yet. The parts are similar, but it's like comparing a Ferrari to a jalopy held together with duct tape. Sure, you can get it to look pretty nice, and you can be a pro at using duct tape to fix anything, but it's still not the real thing, and that just makes my skin crawl. Too much to actually be interested in anything involving those bits.
If we were to go into fantasy mode however, and say that it's actually a fully-functional female body (be it magic transformation, body-swapping, downloading someone's mind into a clone, etc)... then I'd be fine with it. Granted, it all depends on their personality (I'm not fond of overly-masculine women, for instance, no matter if they've been a woman from the start or not), but I don't attach someone's birth gender to anything if the body's right.
On the topic of personal, platonic relationships however, I'm more open. Like I said, the only reason I'm uncomfortable with transgender is because of them not having the right bits, not because of any issues with personality. As long as being with them doesn't preclude me from having relationships with others as well, I'm fine with it.
As an aside, I've heard the term "lesbian in a guy's body" being tossed around before, and... actually, I think that applies to me as well, to be honest. I sometimes think the only thing stopping me from considering a change myself is the lack of sufficiently advanced technology.