I know I'm coming to this discussion a little late, and much may have happened between you since the previous posts, but here are my two cents.

It is possible to engage in BDSM activities without actually "having sex." I mean, it's all related to sex, but many professional dominatrices have boundaries as to what they will allow, and will not include intercourse or certain other contact. Many people engage in BDSM activities outside their marriage or primary relationship with their partner's knowledge, and the understanding that it's the bondage and domination experience that is desired, not sexual intimacy. Now I'm not saying whether this is right or wrong, or appropriate for you, or whether this is the case in Thailand, or whether this was what your husband wanted. I'm just saying that the desire for BDSM does not always cancel out a true loving relationship. Sometimes a person wants this but cannot see their partner in a dominating role.

Having said all of this, a way that you could approach the subject and get it in the open for discussion between you might be to offer up your own fantasies. Even if BDSM isn't one of your fantasies, you might mention it in terms of something you had found interesting and wondered about. A good introduction to the world of BDSM and the appeal of "alternate" sexuality is Different Loving: The World of Sexual Dominance and Submission by William Brame and Gloria Brame. There are other books as well, and lots of informative websites on the net (not porn.) You might even find something that really does interest you as a way to bring up the general subject.

If he wants to know why you started reading this stuff, or how you got interested, you could say that you ran across something on the net that piqued your interest, instead of telling him that you read his email. He might end up telling you about his plans for the trip that he didn't take. Now I realize that this is not complete and utter honesty, but you have to decide the best approach in the interest of improving communication, rather than causing him to be defensive. (I agree with the previous posters' points about the issues of trust, but sometimes you have to take steps and can't get there all at once.)

I'm not telling you what you "should" do-- it's just food for thought.

Pat