Creator Russell T. Davies stated his intent to normalize fluid sexuality across the cast without defining characters by it or injecting politics, producing a program he described as very bisexual. This approach places sexuality as a recurring but secondary element that influences interpersonal dynamics while remaining subordinate to the alien procedural plots.

No established characters receive race or gender swaps, and the sci-fi setting keeps diversity incidental rather than foregrounded as activism. All five main characters are canonically queer by design. Jack Harkness is the prominent omnisexual lead shown in multiple same-sex relationships and kisses, including a passionate kiss with the 1941 Captain Jack Harkness in episode 12. Subplots include Tosh's encounter with the female alien Mary in episode 7 Greeks Bearing Gifts and the developing romantic and sexual relationship between Jack and Ianto. Gwen and Owen complete the set of characters established as queer by season's end, with these elements shaping arcs without lectures on systemic oppression.