I thought this would be the natural way for Martin to be involved: he's a good friend of the MC and Martin they'd all be out eating some night after he arrives, some drinking is involved, D+MC+Elena have histories of risque games but now Martin is there . . . I could see those games turning into a Martin+E and MC+D coupling that transmutes into a set of Martin+D and MC+E pairups or orgy, all in one crazy night. Or, Martin + D just happens the morning after such a night where it only went as far as D being with MC as her male partner.
I am ALL FOR seeing D's reaction to experiencing sex with other guys and generally in a grouping of three or more. She's already highly sensitive and expressive during sex and climaxing, imagine how she loses herself in the moment of a first-time double penetration!