MMF by definition will include F in the action.
I wouldn't expect to see any D and Martin action without F being an active participant- there would be too much outrage from that staff note in yellow on the 1st post of this thread.
As to when we see it- not until CH4 is my guess
As far as I see the "signs" plus some comments of some of the dev team, any action of D with other men in the future will be with the full consent of F. It's going in the direction of an "open relationship", a more mature kind of love, where the man doesn't see the woman he loves as his exclusive "property" but wishes to see her happy, even if she being happy involves her curiosity and, with time, even her pleasure in having sex with other men.
If he gives her green light to fuck around, she will most probably do just that.. But she will always let F know what she does or did, she will not do anything behind his back, she will not "cheat" on him, she will never hurt him! (although , you know .. never say never.. a little "temporary drama" is always welcome in a love story, lol)
That however doesn't mean that F must always be part of all D's interactions with other men.. Sometimes he will be there, by the nature of things, like when swinging with another couple, (although, well.. they could also chose to go in different rooms
) , some other times he won't be there.. She still has green light once given! And she is curious by nature.. and by F's education
. So i do see a good chance of a single sex scene of D with Martin, before or after a MMF. As well as D alone with other men in the future..
A MMF with F and Martin must be because that is what we should get if we made that choice in Ch1 that started the MMF path and if we didn't cancel that with any of the late choices, but I can see no reason to exclude ALSO the possibility of a D/M single sex scene. After all, D's dreams about Martin were not 3some dreams. Maybe just a BJ with Martin to begin with, and then F joins or.. after the MMF when F may get tired and go sleep and D continues with Martin alone, or.. on another occasion, like when training with Martin for instance.. Nothing speaks against it!
However, Mr.Dots being a well known "slow hand mama" in terms of story development, I would not bet that we will see any of these in the update that is about to come (but hoping costs nothing so .. i do hope
)