Big Deal by Helen Sandler

Lane and Carol have a deal that lets them play around with other partners. But things get out of hand when Lane takes to cruising gay men, while her femme girlfriend has secretly become the mistress of an ongoing all-girl student orgy.

The fine print in the deal they've agreed on means things can only get hotter. As Lane ventures into London's male cruising grounds, and Carol covertly breaks her side of the bargain, it's time for a different set of rules – and forfeits.