Some examples of this in terms of 1970′s costumes are swingers, disco dress, and "pimps" in the Superfly mold. Swingers of course, are defined thusly by wikipedia:

Swinging, sometimes referred to as the swinging lifestyle, is "non-monogamous sexual activity, treated much like any other social activity, that can be experienced as a couple."[1] The phenomenon of swinging (or at least its wider discussion and practice) may be seen as part of the sexual revolution of recent decades, which occurred after the upsurge in sexual activity made possible by the prevalence of safer sex practices during the same period. Swinging has been called wife swapping in the past, but this term has been criticized as androcentric (taking a male-oriented point of view) and inaccurately describing the full range of sexual activities in which swingers may take part.

Later, wikipedia goes on to state "In the United Kingdom there was a proliferation of neighborhood groups in the early 1970s — known as "wife swapping" groups — and press articles in later years suggest the peak was 1973–76."

So chosing a swinger outfit as the best of the 1970′s costumes makes some sense. Same with a disco outfit – just look at "Saturday Night Fever" for inspiration, since that is the definitive disco film and provides the best ideas for 1970′s costumes of that mold.