I am sure many people will have many varying views on this.
First off, I am very sorry for any pain and wronged feelings you have towards your partner and the lifestyle she has embraced. I am by no means a 'voice for the people' the following is merely my humble opinion as a life long member of the poly community.
All relationships, and especially Poly and ethically non monogamous relationships should encompass the following things.
Trust
Honesty
Communication
You have had your trust knocked, been treated dishonestly, and have had a total communication break down. These are not easy things individually for anyone to bounce back from, let alone suffering the triple flip.
Some people are built for monogamy. Some people are and always will be a one love person. Others are built differently and the idea of 'more love' isn't such a scary and ethically questionable concept. In fact, it just feels right. Often when these two types of people collide, someone will always lose, or be forced to hide who they are forever.
I have always believed that you can not be honest with other people until you are honest with yourself. It sounds as if your wife if being honest with herself about what she wants, but has neglected to pass that on.
Do you love your wife? This is a complicated question. What I mean is, do you love the person your wife is, not who she was, not who you want her to be, and not who she may be one day; but rather do you love her as a human being, a person you have shared years with, and someone who knows you better than most? Do you love her right now?
She is in the wrong, no doubt about it. You both made a commitment to Trust, be Honest, and Communicate. You made that commitment as well. It is your turn to approach her and talk about what has happened. Farnkly, and honestly.
In a word, be the bigger man.
It is not impossible that your wife has had trouble finding the right words herself, that she has taken this long to approach you out of fear. Maybe what she needs is you to tell her you are willing to listen. (This does not mean you have to agree)
Many people who are into BDSM, and many in the poly community have children. I can understand your fears about her levels of privacy. It is possible that with a few request from yourself she can tone it down a little. It is not uncommon for people who have chosen to embrace a new life style to attack it with vigor and perhaps a little too much enthusiasm in the beginning. It's a common mistake to make. I am not at all surprised that she has connected with people in your community. There are many more people in alternative lifestyles than popular culture an society would have us believe.
Do you, yourself, have any interest in sexual activity beyond the vanilla ('Normal')? There are many things that could become a fetish. Some of them are as simple as a certain position or a location. Fetishes can be fleeting or a requirement for sexual enjoyment. If you do have one, talk to her about it, use it as a jumping off point for her own expressions of sexual desire. Either way, people should not be ashamed to discuss their desires with their partners; but understand that it is ok to say 'No'.
I think the key to going forward for you is the rule of Communication. You have a duty to be honest when you communicate with her, and in that honesty, I hope she can find it in herself to do the same to you, so that both of you can start to rebuild trust. This doesn't mean yelling, screaming, or crying (even if all these things can be part of your communication); it means taking the time to sit down and address your feelings.
None of this, nothing she has done, is a reflection on you as a person. This is ultimately a choice she has made for herself. This may be rather selfish as a person looking in from the outside, but remember that she is experiencing her own emotions and changes. While the pain my be different, she may very well be hurting also. She has chosen to make the first step and talk to you about it, show you some of 'who she is', when she could have just as easily not. She clearly cares about what you think.
I wish you all the best. I am sorry you had to be introduced to the lifestyle in such a heavy handed way, but know you aren't alone, and that many of the people who have now embraced it were once bit by it. xx
All the best <3