Hello- I see that you are offline and I will respond to your question and you can reply if you wish.
People that are transvestite are not mentally ill. Rather, they have a fetish for dressing and/or behaving as a women for a number of reasons. What is important for you is to learn as much as you can about transvestites and then come to some understanding within yourself as to whether you can tolerate the behavior in your life. Some people can and some cannot. It is a very individualized issue.
I am going to give you a couple of links to help understand this from your boyfriends perspective and also from those who are in relationships with transvestites.
I hope this helps you address and deal with this issue. Please reply if you have questions.