I understand that this was a shock but take a deep breath close your eyes and count to 10. Well maybe a 100.

Now the good news. He loved you enough that he admitted what was no doubt something he has hidden from everyone his entire life. A secret that he may have wrestled with for a long time.

You husband sounds like a mastacist.
noun 1. Psychiatry. a person who has masochism, the condition in which sexual or other gratification depends on one's suffering physical pain or humiliation.
2. a person who is gratified by pain, degradation, etc., that is self-imposed or imposed by others.
3. a person who finds pleasure in self-denial, submissiveness, etc.

This is different from a sadist
noun 1. Psychiatry. a person who has the condition of sadism, in which one receives sexual gratification from causing pain and degradation to another.
2. a person who enjoys being cruel.

It is important to remember that this is not a different man then the one you have always loved, you've just discovered a different facet of his personality.

The bad news is that fetishes in my experience tend to be part of us for life. Personally I can deny my fetish for a time (sometimes years) but the longer I do the stronger the pull becomes when it comes back. Denying it is denying who you are it would be like being gay and marring a woman its not healthy. There is the possibility that therapy can help discover the root causes as to why he has the fetish but it rarely if ever cures it.

The best approach is to take time to look at S&M from a researchers perspective. Don't just type S&M in Google or you may be shocked. Acclimate yourself to it. There are asspects that may not be so shocking such as spanking, blindfolds, tying up, calling him names. If you are comfortable with some of these discuss that you would like to indulge his fantasies but lay down very specific rules or boundaries. If you do this it may help curb the urge to look elsewhere to fulfill the urge. In fact you could use this opprotunity to role play humiliating him about it.

If you can't indulge it then its perhaps best to let him continue as he was doing. Follow a don't ask don't tell policy as it were. If you're unable to do this then it would be in both of your intrest to see a marriage councler and see if they can help resolve any issues.

The last thing and I wouldn't do it without talking to him first but he may like being humiliated by telling a friend about it.

I wish you the best of luck. Just remember this doesn't mean he doesn't love you.