Teaching children about gender/sex/sexuality.

Sa
To everyone who has kids or is planning on interacting with children in the near future, I want to know HOW you are going to teach your children about sex, gender and sexuality (related moreso to the trans and gay community, etc).
I babysit for the most adorable 5 year old boy who understands of course that I am a girl, mom is a girl, dad is a boy, etc. But he still needs it reinforced so he asks and jokes about gender all the time. While watching tv, "is that a girl?" He'll ask. Or when I say something vaguely related to my gender, "because you're a girl?" To which the answer is sometimes yes, sometimes no. And lately he asks all the time about marriage. Will I marry a boy? Will he marry a girl? Daddy married a girl. Etc.
This family is a Christian family, and I someday will raise my own children in a Christian home. I want to be inclusive about all genders, sexs and sexualities at some point, so when do you start explaining it to a child? I'm 20 and I'm a cis female (?) who STILL gets confused and lost among all the relatively new labels we now give ourselves and others regarding gender, sex and sexuality. I was raised in a home where females were girls who grew into women and males were boys who grew into men who married the women. With this way of thinking I still feel attached, but times are changing so I want to know how you all plan to incorporate this into parenting.