Hi,
nice to see that you are back :-)
I would give a small morale boost, then subtract morale for witnessed/participated lewd actions. I think this would be better both w.r.t. balancing as well as immersion.
Balancing: If I remember correctly, the morale loss when meeting a sleazy male officer at level 2 is 2, while at level 4 it is about 5? So the difference is 3. Missing your evening relaxation is huge, especially as you cannot avoid it. When you reach relationship level 4 with some officers, you normally get 6-8 morale each evening. Stern female compensates that by having a reduced morale drop for missions, sleazy female doesn't have this. Plus, I guess you can still get the level 2 scenes in the office. So an initial morale boost might be good, it is still worse than getting your normal evening program. Depending on how fast you will get penalties for lewd actions, I would give an initial boost between relationship*0.5 and relationship.
Immersion: If you are invited to the private party of a girl that you know she has a crush on you, but you don't share her feelings can be an akward situation. But you can still have fun at this party, at least more fun than having a bind date with a creepy stranger. However, as soon as you have to do some dares on this party, this might change...