When the receiving sexual partner, typically a female, is intentionally put inside a box for sexual purposes. The box is usually shut in such a way that the person inside can't escape without assistance, and there is usually a small air hole and a bigger "Sex Hole" where the vagina/anus is located. A Sex Box is normally used in a situation where a receiving partner wishes to be "used" by multiple people, and the box allows them to both be traded like "property" and allows the person on the outside to "use" him/her without caring about their looks, since all they see is the vagina and/or anus. Some variants include a mouth hole, and more permanent housing.
Guy 1: Where's your girlfriend?
Guy 2: Her and I had box sex last night.
Guy 1: So is she still in the box?
Guy 2: Yeah.
Guy 1: Really? Can I borrow your sex box?
Guy 2: Sure, I don't mind sharing her.