Hi Will and thank you for your question.
It is okay to feel confused right now as these are some feelings you are unsure of and are questioning.
I understand that you consider yourself to be a male and are comfortable with that. However, you present the question that you may be transsexual because of your fantasies in which you are a submissive lesbian. You add that you don't have the desire to dress up in women’s clothing and you are comfortable with your body.
The definition of transsexual is an individual who is born physically with the body of one sex but mentally feels like they are another gender. For example, a person who is transsexual would have all the physical characteristics of being a male yet psychologically he would feel like he is a female. Many transsexuals feel that they are imprisoned in a body that they do not identify with. This divide between what a transsexual looks like and what they feel in their mind is present throughout most of that person's whole life, not just on selected occasions. If you had constant feelings of being born in the wrong body, you would fall into the term of transsexual.
Another term you may have heard of is transvestite (sometimes called cross-dresser). A transvestite is a person who dresses up in clothing typically worn by members of another gender. Some transvestites only cross-dress for certain events or times, while others do so all the time.
It is common that transsexuals, who feel like they are in the wrong body, will dress as the gender they identify with.
Yet another term used is transgender. This is a blanket term used to include an individual who is transsexual, transvestite, gender queer or any other category in which members have incongruent feelings about their gender identities.
Referring to your fantasies about being a lesbianfantasies may not always reveal who you are. Even though you have these thoughts about being in a sexual encounter as a lesbian, it doesn't necessarily mean that you want to become a woman. For everyone, fantasies may just be a break from reality, a place to explore the unknown or to blur gender lines that would otherwise be left alone. This is normal. Some people have sexual fantasies that they would never indulge in when in a nonsexual environment. There are people who get turned on by imagined encounters but never actually partake in them in an intimate setting with a sexual partner (or partners).
The term male lesbian can be used to identify men who desire to be a lesbian. A common belief about male lesbians is that if the man in question were to be born female, they would then be exclusively attracted to females (lesbians). Being a male lesbian can have numerous meanings to it, but it does not mean that this particular individual is transgendered. I find that this term is relatively unknown and doesn't have as much research on it as other mainstream terms like gay, lesbian, bi, trans, pansexual, etc Male lesbianism has been explored in the Showtime TV series The L Word in which a male character embarks in a lesbian relationship with a woman. However, this reference can be considered fiction and some aspects may not be correctly portrayed. However, it IS something you might want to research further.
Remember, those who live day-to-day life believing that they are in the wrong body would be considered transsexual and not necessarily those who have fleeting fantasies about being another sex. Terminology created to describe a person's sexual interests should not bind you down to living in accordance to that definition. You are unique and can pursue any path in order to seek out personal happiness.
If you wish to read more about transsexuality, or male lesbianism, you can refer to the links below.
If you have any further questions, please don't hesitate to come back.