So first big actual story stuff so I'll drop some in-universe explanations here as a tad of insight

No1 what the **** is going on
The simple explanation is after a tough night dealing with Meltdown symptoms Luna was struggling pretty badly so Flack (who has also been in that situation before) offers a form of therapy known as core burn therapy, after an hour Flack suggests he leave to avoid making Luna uncomfortable but she protests and he stays with her and they both eventually fall asleep

No2 what is Core burn therapy
A form of therapy found to calm the meltdown nodes of frames by exposure to another frame’s warmth without physical contact, Flack learned it to treat himself but also knows how to implement it with others

No3 the hell is a meltdown/meltdown node
A meltdown node is a form of commonly occurring creation defect where extreme stress or trauma can cause curtain behaviour to worsen, a meltdown is when the meltdown node is overwhelmed causing not only eminence distress but also worsening the controlled behaviour to critical levels, putting those around them in danger (it's due to this that many fear those with meltdown nodes and gave rise to the horrible procedure that is known as Fixing)

No3 Why does Luna sleep “nude”
It's a cultural thing plus she knew she wasn't in any danger as Flack is under oath still at this time and can't even retract his pelvic cover (Luna had hers on two so literally nothing could have happened)

No 4 oath, Flack, WHAT
Flack was once a Templar who was ordered to protect the Owl class frames at all costs, While under oath Flack can't form sexual relationships, can't get engaged or bond his core (basically marriage) and can't form another oath

And finally, No5 Sex! b-but they’re robots aren't they
Yes you can blame humans and their belief that reproduction must mean fuckin, even frames don't know why they have to as external fabrication is something that can be done for same-sex families to have their own children (so basically their human designers believed that humans had the most superior reproductive system and copied it for frames)

If you have any questions just ask :)