Martin won't be the initiator, he'll probably be the last one to know. D will probably initiate it.
I posted a while ago about a potential plausible scenario
I think a fairly simple and realistic buildup would be when he comes to get his van, D/F and him go to a restaurant or something and Martin lets D drive the van (didn't she already do that?).
- Go to a restaurant (to chat, catch up with each other etc.)
- *Choice* D drives the van
- *Choice* Perhaps Martin sits in the backseat behind D and puts his hand on her shoulder to encourage her.
- *Choice* D and or Martin flirt (about the driving skills of D for example)
- Martin spends the night, which would lead to the sharing if the player didn't reject the previous choices.
To expand on this more
Martin comes over to get the van -> MC/D ask Martin to go to a restaurant to catch up -> At the restaurant, D can sit beside Martin or MC (sitting beside Martin is the first step to sharing) -> When they leave, Martin asks D if she wants to drive (yes or no) -> They get home, and watch a movie or play a game -> Then the sharing happens.
The simplest way to get the actual scene is probably by playing a game. Similar to the twister game D/F and E played. There isn't a second bed in the apartment, right? Could let Martin share the bed with MC and D on the sharing route, and he stays on the sofa if not on it.