I didn't want to comment on this, but felt I should.
If being with the same sex person is not your thing then you should be honest and get out now, even though you have a great job you won't be happy if same sex relationships is not your thing..
If you were undecided about your status then make sure your boss knows this. Things could get ugly if it all goes wrong.....
By the way, romantic trip and london, england don't belong in the same sentence....imo.