[spoilers] Lucas. Just Lucas.

[Poppy]

I found episode 8 of series 9 so sad, especially on the roof with Harry because I think they both felt they'd suffered the ultimate betrayal and the betrayer felt no remorse. Maybe for Lucas it was too much and that was what (literally!) tipped him over the edge...

As for the Maya/John relationship maybe the whole intention was that it was so strong that it was beyond physical like with Sarah, and you go by the fact that she trusts him completely and he will do anything for her...
I'm not sure, just a guess