Back at home and alone for a moment.
I really do appreciate all the responses. Being able to share my thoughts and feelings with someone helps more than I can say!
I'm trying to keep an open mind for the time being. I'm happy to be in a relationship with someone who's bi - I have been before (he cheated with other women - different story). I'm also happy to remain friends with someone who's gay - although I admit that losing my lover of eight years would cause massive heartbreak no matter what the circumstances.
For what it's worth, I've decided to stop trying to slap labels on DH for the time being. I subscribe to the view that most people's sexual orientation tends to be some shade of grey or other rather than pure black or white. He is whatever he is. I guess it's for him and me to figure out what each of us wants from a relationship and whether our wishes are I any way compatible.
The bit that really makes me hurt is that I didn't know, did not get any chance to decide whether I was up for this ride (and I might well have been). And then, of course, there is the thing about my own gender issues. The fact that, no matter how well I've been raised by my radical feminist mum, I have still internalized certain ideas of what a "real" woman is and how she has to look and act in order to land and keep a man. Not something I believe in on an intellectual level; however, emotionally, I suffer from a sense that I have failed as a woman. Which is admittedly fucking dumb!
Decided to ignore the above admission for a while, slapped on some serious makeup and spent a bit too much money on some gorgeous lingerie and a pair of ridiculously expensive heels (purely for me, not DH!!!). Probably not the most intellectually coherent response, but it did help. Now resisting the urge to feel I've also failed as a feminist. 
I'm not really sure yet about some of the questions that have been raised here, e.g. what triggered DH's decision to let me know. I am planning to find out, of course. I don't think that wanting to sleep with another man is it, really. I've always said to him that if he would ever want to try this I would be fine and vice versa. None of us has ever asked for the free pass and he still hasn't.
Basically, I feel very ambivalent about the entire affair. On the one hand, I'm happy he's told me, pleased that apparently I'm the sort of wife a man like my DH can tell this to and - on an abstract level - very, very proud of him for coming to terms with his own feelings and expressing them.
On the other hand, it's also kind of as though someone died. I suppose grieving really does come closest to what I am feeling at the moment. And I'm angry because I feel deceived, insecure, because my reality has just sort of rotated by 90 degrees and hurting.
Sorry, this was much, much longer than I intended for it to become. And there's a lot of rambling. The thing is, I don't feel very coherent at the moment.