Craig has stumbled onto a group of women who call themselves the BiGirls. They are bisexuals who enjoy giving pleasure to each other as well as to their husbands or boyfriends; and they take pleasure from each other the same way, and see no harm in doing so. But their husbands might not understand. So they don't want Craig snooping around to take pictures of them to use to blackmail them into not blackmailing him, so that he can blackmail them into selling insurance to their husbands. From now on, when they're meeting, they'll want to know exactly where Craig is. And that'll be right there with them at their little sex session. Thus does Craig become an honorary member of the BiGirls Club.
Kinda catches your attention, doesn't it?
Of course there are catches and conditions. For example, Craig has to wear female clothing during the meetings and has to act as much as possible like a woman. This may sound humiliating; but the way the author presents it, Craig actually enjoys it immensely. The activity even improves his sex life with his wife. A Club rule is that a BiGirl who is unoccupied at a meeting can never refuse another member's request, and Craig soon begins looking for interesting things to do with each of the other ladies. Anything at all is permitted, as long as it is something one woman can do with another.
Oh - and Craig also takes medication, so that his erections become fewer and softer. At BiGirl meetings all danger of inadvertent penetration eventually ends. Craig progresses very well, and the other girls practically applaud the first time he takes a dildo up the ass. The other girls become openly affectionate toward Craig. The experience brings out his feminine side - making him a better person.
Eventually Craig is seriously considering going for full rather than honorary membership in the Club (without surgery, of course); but then his wife throws in a monkey wrench, so to speak. Things get very complicated. As usual, the last quarter of the story is full of surprises; and this author does a great job of making even the most bizarre events seem perfectly sensible by the end of the story.