If it's permission, then it's not cheating. Cheating is when you do it without consent. Yes it's still an affair but that's a whole different animal and can work if she's open about it.
And I'm in total disagreement on this. Having an affair with a woman as long as it was with my consent and in full transparency would be fine with me (but not another man.) I'm not saying it would be without potential problems but I could make it work. Maybe it's because I married a bi-curious woman and she was open with that from the day I met her. I loved that she shared this with me. She never did pursue it but came close and I would have been just fine if she did.
Maybe that's just me.
It is not just you - my wife is bi-curious as well. We have discussed it and if she wanted to do something with another woman (with or without me present) I would be fine with it, but that is just me. I can't explain why that is, it just is. If someone faults me for that, that's fine.
However, many people feel that it is cheating regardless of the sex, and regardless of whether or not they have consent from their spouse. That is a valid point of view as well, and I don't fault anyone for feeling this way.
Cheating is cheating. Period. I never said he gave consent, I said he was knuckling under to it. Once the vows are taken and sworn to, there is no wiggle room. It doesn't matter if it is a woman or another man, it is unacceptable, and it is infidelity.
If you want to screw other people, great, the answer is simple. DON'T take vows of fidelity to one partner. If she wants to test out her bisexuality, she needs to divorce and go for it. But as is, if she does this, she is a cheater! Plain and simple.
If she feels this is her destiny, her husband should divorce her, wish her luck, and find a woman worthy of his time and effort.
Personally I don't think the problem is that she wants to. The problem is that he doesn't want her to, and she is pressuring him to do it anyway. Or maybe he is not voicing that concern to her, and that is a problem too.
However, I don't think it is a good idea to imprint your own values onto someone else's relationship. I don't agree with the OPs point of view, but I think it's important to acknowledge it and give advice based on his values, not mine.
To the OP - I agree with other posters here in that she doesn't seem to respect your marriage. Believe us, she is likely to go ahead and do it behind your back. By forcing yourself to give consent when you don't really want to, you're giving her an out - "But I thought you were OK with it!"
Doing something she's never done to "get it out of her system" seems like flawed logic to me. How does she know she won't like the other person more than she expects? If she then decided to leave you and take half of your assets, you can't really say anything because "you said it was OK!"