I think Elena ist the most logical next one especially to do more kinky stuff. An D/E/F threesome would fit. To open the M route more, E could be a catalyst. Maybe D and E talk about D's fantasies about M or E talks about BBC and how she would love to get banged by one -> M. Maybe D watches how E seduces M. So this could lead to a D/M sexual encounter.
Didn't we already have an initial threesome among them, albeit more of Elena participating by mostly directing the other two? I agree that would be a natural next step, at least in terms of no-holes-barred participation . . . but now Jennifer is with Elena and Jennifer is definitely not one to be left out of a given opportunity at this point. It could end up even as a foursome, where the MC takes turns with each of the young women, while the other two either do their own thing nearby or help out the MC and his current partner, maybe.
I could see Elena adding to D's interest in having sex with Martin. I don't know if D would mention a specific person to Elena or just a curiosity about interracial sex in general.
The bigger issue is D talking to F about her wanting to have sex with Martin. F already knows D has a school girl crush on the photographer, but the Martin thing takes him by surprise.
Of course maybe Martin will discuss his attraction to D with F first, and F confronts D about her feelings toward Martin. This could give D permission for guilt free sex.
Honestly, I was expecting Martin to either reason from clues or otherwise catch them in the act, either path helping him realize they are actually a couple. And, that then leads him into the circle of acceptance once he and the MC come to an open understanding of what's been happening, which then leads to more open talk of other possibilities.
That could all complement the progression of discussions you mentioned, I feel.