Gender and Biological Sex are two separate concepts. This is how I see them.
What one feels in their attitudes and overall expression, let's go with Feminine as an example, are those that are common among the gender known as Female. Females, have feminine qualities. And Vice-Versa with Males, having Masculine qualities.
You can now be free to express your inner feminine/masculine qualities that you identify with in the manner that presents that quality. This expression is in the form of dress, behavior, emotions, and even dance.
Societies have assigned, and this is in nature itself, Gender Roles of Masculine and Feminine energies. Which is an entirely different concept than just being about males and females.
Biological Sex, has to do with the physical body one was born with, or was given at birth. Nature determines that Masculine energy usually has a penis attached that develops sperm to impregnate, Feminine Energy has a vagina that receives the sperm and forms an entirely new human body (in some species, their biology of this is reversed, or not ingrained in their biology at all). This is the process of life, it's a gift. And it's a big reason for why Native Culture hold women as sacred, and a reason for proving why our culture is matrilineal. In our society, the men didn't really own anything, nor had a say at all with feminine women matters such as child birth. The home was owned by the women, made by the man for her, just the same as a womb is the home of the fetus, so is the home a womb to shelter, comfort and soothe the human. The fields were also owned by the women, Just as the uterus is the field in which the man plants his sperm, so is the soil in the field the uterus of the Earth that man plants his seed into to nurture and grow the foods we need to survive.
So you have humans whom were born with a Male body, whom have Feminine Energy within. And a Female body, with Masculine energy within. Within some societies, these people had their own terms for these people....
https://www.ihs.gov/lgbt/health/twospirit/And they incorporated themselves into both gender roles and positions. Which involves more than just the nature of Sexual Relations with another human of the same sex.
This is how I define the terms of "Cis", "Straight", and "Heterosexual" in relation to their counterpart terms of "Trans", "Gay/Lesbian", and "Homosexual".
Edit: I wanted to add...
Humans are now able to change and manipulate their naturally given body, to mold and match the energies they are projecting. There are also humans whom are fine with being a feminine energy within a man's body, and vice versa. They would be called a "Cis-Gay Man" or a "Cis-Lesbian Women", while the former would be "Trans-Lesbian Women" or a "Trans-Gay Man", as their inner feminine/masculine energies are now identifiable by transitioning into their inner self by manipulating the outer body to reflect that inner self, this is what "Trans" is to me, while "Cis" are those who are happy being that inner masculine/feminine energy in a male/female physical body.
I would identify myself as a "Cis-Bisexual Man" meaning I am a man whom has a male physical body and likes to engage in relationships with other male physical bodies. I am not really attracted intimately much to highly effeminate male physical bodied men, or Trans-Women. Being Bi, if I want a feminine body I will go after one with both Feminine Energies with the female body.