I would not stay silent.
I would not leave.

My guess is that he believes that you would not want to roleplay out the BDSM fantasy. He may be ashamed of it.
May peple who have these fantasies have shame about them. Not with strangers, but with their loved ones.

I would sit him down and let him know a few things.

1) you read his email. That this was wrong if he expected privacy.

note: if he feels scared or trapped or immature, he many make a mountain out of this molehole. Let him do that.
Some people are very touchy about boundaries and betrayal. The fact that he is planning on cheating does not excuse your misdemeanor.

2) let him know that you know about his fantasy.

now comes what only you know.

If you refuse to at least try it, then will you give him your blessing?

If it were me, and I am not, that I know of, into BDSM, and I did not feel comfortable about doing a "scene" I would suggest we go together to some S and M events as observers.

You both need to see a marriage counselor with an expertise in sexual issues, and/or possibly a sex therapist.

Does it make you sad or puzzled he would refuse to share this part of his life?

Let him know you are willing to work this thrugh.

I hope this helps,

Prayerfully,

Pete