Swinging has switched from a male to a female focus:

Apart from the numeric growth of the phenomenon, swinger behaviour has also greatly changed over the last 25 years. Both in Italy and in the rest of the world. Twenty-five years ago couples sought almost exclusively other couples or single females; now a days couples seek other couples, single males and groups of males. The phenomenon has thus evolved from situations in which the centre of sexual intercourse was the male component of the couple to “harder” situations in which the centre of sexual intercourse is the female component and the male takes his pleasure from sharing his companion with other men. … Now a days an increasing number of single males seek swingers rather than prostitutes, since swinger women are considered more participative than prostitutes.

A swinger party price list:

Couples: $90, MFF Trios: $120, MMF Trios: $170, Single Females $25

Some patterns:

• Swingers tend to have at least one person per couple with a college degree.

• The typical rule is that it must be couples, no singles allowed.

• You must first present a negative STD test, and agree to be routinely tested.

• Many have restrictions on outside sex … you [may not] “cheat on the group.”

• [First talk] on the internet for a long … time before anyone will approach you.

• [Then] several “dates” where they meet the new potential couple.

• Groups also sort themselves according to their preferences for activity types.

I’m not sure I’ve got the right picture here – not even sure these are the same kind of swingers.  When many men are on one women, the other women are just sitting around being ignored?  Back in the day when men were the focus, were several women on one man while other men were being ignored?

My guess on the focus switch is that women less fear seeming slutty today.  Whereas once swinging women might have been with several men in one night, they drew the line at being seen doing many men at once.  Now that line has moved.

Added 9a: Apparently, men in couples mostly initiate the move to swinging, but some women become very enthusiastic converts, and become the female centers.  In the past such women were less visibly slutty, with one man at a time.  Many men are overconfident, and learn they attract few women at such events.