When D and the MC talk about what happened with Elena the next morning the MC informs D about other forms of multiple partner sex activities one of which is MMF. In D's internal monologue she wonders about how the mechanics would work for the girl so it seems she is already intrigued with the possibility. Let's just hope she takes her "pill" that day.
This still does not address the elephant in the room, how is Martin going to be informed about D and her father's sexual relationship?
Ironically Elena had more success this update getting her girlfriend to preform anal than does the MC, maybe she can give him a few pointers.

Hmmm, could Elena be the first one to have anal with D?
After few great updates, I was definitely expecting more out of the last one. Almost like natural progression stalled.
For one, anal didn't really happen.
Disappointed that D and Elena didn't go to nudist beach. That could have been great setup for orgy or sharing route.
D and F, possibly with Elena, could have watched MMF movie including DP. Elena would have found it cool, and peaked D's interest.
Next day Martin arrives, and after few drinks, D, F, Elena, Graham, Olivia and Martin all go to deserted nudist beach in the late evening, when is already getting dark. At the beach, they could possibly encounter Margo and her BF, who were unbeknownst to them, spending their vacation in Seaside.
This would allow for more natural setup where everyone is nude for a reason, and after few more drinks, everyone would get loose.
Afterwards, they could have changed venue, possibly involving swingers club.