I mean, they were planning to have sex anyway, because they have that pill which is supposed to stop MC from turning into a ghoul. The fact that hibiki think it's not dangerous has no impact at all.
Your points about mind slave or other such things might be valid, but only if the MC makes them, which he didn't. You can't make up things to explain the story, the story needs to do that itself.
In that specific instance what exactly do we know about Hibiki?
Correct.
That she is the mother of Jun and has serious mental and marital problems. Maybe the last two are linked, but this to the knowledge of MC is speculation.
So why pray tell should the MC mindlessly act immediately upon such an untrustworthy order?
Even if she did not have a "memory loss episode" right when she apparently went on to explain the above question stays the same.
Keep in mind while the MC has a severe case of helper syndrome he is not stupid.
To the contrary the talk of Hibiki is likely to plant doubt whether having sex with Suzu is a good idea after all.
It could be that Hibikis order was "programmed" into her by the enemies of mythkind.
He basically was lowkey warned by the headmistress before she hurriedly left for europe for her gathering vital information about the issue.
The pills
could be harmless.
The pills
could be effectless.
The pills
could have adverse effects.
Thing is it is basically an experimental drug which is not really that much understood by the scientists, but they
believe it should work.
He has not taken a single pill yet. He does not even know how they taste.
That MC is not human but not a "full Myth" either is pretty much confirmed. He pretty much is a special case himself.
Reina previously said that MC's genes are different now from what they were a week prior when he was her patient with a smashed skull.
If and how the experimental drug that works for most agents will effect him is unknown.
Maybe the pills "only" prevent him from turning ghoul during/after sex with Suzu.
Maybe he transforms into a sex starved incubus, rounds all girls up and fucks them into oblivion and back.
Maybe his brain explodes 30 seconds after taking a pill and/or filling her womb.