There are many things in our lives that we don't share with our life partners. How many other women we have slept with and how often is a good start. The fact that you fantasize about banging her sister or friend is another. The fact that you "pleasure yourself" while looking at porn would qualify.
If you are a serious crossdresser and are starting a new relationship, this would be a good time to tell her about it. If you have been together for twenty or thirty years and your desire to dress is more recent, it's hard to bring it up for fear of losing her or seriously changing or damaging the relationship.
Yes, trust is important and I trust my wife not to tell others about Krisi, yet we are still individuals and some things remain unspoken.