@ OP :
I feel bad you`ve been labelled as some kind of homophobe.
Unfortunately, in this day and age, if you aren`t all rah-rah-rah-horny over something, it MUST mean you are 'phobic'.
Seems a simple case to me, of you feeling pushed into something you don`t want to do. Substitute BDSM where bisexuality is, and everyone would be bashing your SO for trying to pressure you into something, you don't want to do.
I do believe, however, that your SO is basing his ability to feel accepted, and lose his feelings of 'wrong', on YOUR reaction.
Thats not going to work. He needs to learn to accept himself for who he is.
You do need to be understanding, but not at the expense of your own abilities. Do some reading, and research. Find things you can do, to help him feel like he isnt less of a person in any way. help him, help himself though. Not lean on you for 'proof.'
I would also suggest to people reading this,..sheesh, give her some time. TRUST is a huge part of any relationships ability to move forward. She is just learning something about her SO. Wether he intentionally, or unintentially kept this hidden, doesn`t mean her ability to work through this is going to be any quicker.
Right now she seems to be having a gut-check. That is a good thing. The fact she is asking questions, means she wants to figure out a way to handle this.
Let her get over the fact, that she feels like she is just getting to know him. Then when they are back on track, she might be more open to other things, who knows ?
...and I second Mags approach. PM`s might be more beneficial at this point.