The usual use-of-Sailor-Moon disclaimers apply.  Other notes at the end,
since they include a spoiler.

******

Ami scanned her book as she walked, quietly musing at the irony of it all.
She had long since developed the talent of reading and walking at the same
time.  It just so happened that she was reading her biology text, and had
just reached the chapter on reproduction, when the other Sailor Seishi
decided they had waited long enough for Usagi and Mamoru to show.  The two
lovebirds had said they just wanted to spend a few minutes alone, but that
had been half an hour ago, and Ami had drawn the short straw to retrieve
them.  As she read a brief paragraph about mating calls, Ami wondered if
she would hear their cries of affection.

She nearly dropped her book when she heard Usagi moan.  Cautiously
stepping towards the noise, she peeked through a door that had been left
slightly open.

Beyond the portal, Usagi was in Mamoru's lap, lost to bliss under her
lover's caress.  Usagi's entire body was limp, with her eyes and mouth
closed as she concentrated on something.  One of Mamoru's hands stroked
her chin, while the other supported her thigh.  From the smile on his
face, and the bulge in his trousers, Ami could tell that he shared at
least a fraction of Usagi's pleasure.

As Ami watched, Usagi stiffened, arched her back, and moaned louder, but
not so loud as to put Mamoru off.  Her lips and eyelids stayed closed as
if glued.  Ami could see goosebumps on Usagi's flesh, which served to
highlight spasms of what, judging by Usagi's expression, could only be
ecstasy.  Although her mouth permitted no air to pass, Usagi took in
shuddering breaths through her nose as her fists, dangling helplessly to
Mamoru's side, clenched and unclenched of their own accord.

After what seemed like a lifetime, but was in reality only a few minutes,
Usagi calmed down.  Hesitantly opening her eyes, she looked deep into her
mate's eyes and soul, and quietly pleaded, "More?"

Mamoru smiled, and bent to kiss Usagi as he reached for a heart-shaped
container of brown chunks...

"What are you DOING?" Rei hissed as she jerked Ami away from the door,
just silently enough to not be heard by the lovers.

Ami stepped away from the door under Rei's forceful guidance.  "Well, you
said I should get them..."

"Not in the middle of this!  Besides, you weren't getting them, you were
just standing there."

Ami blushed, and continued walking back towards where Minako and Makoto
waited as Rei scowled at her.  A few seconds later, she commented, "I'm
jealous of Usagi."

"Why?  You were never in love with..."

"No, no.  It's just, well...I don't think I'm physically capable of what I
just saw."

Rei was about to ask, but thought better of it.  Although she was curious,
she could just listen when Minako pumped Ami for details when they
rejoined the group.

Nevertheless, Ami continued.  "I mean, sure, I like chocolate, but not
THAT much..."

******

I got the idea for this from something that happened in real life.
No magic or (non-explicitly-stated) drugs here; just Usagi and two of
her loves.

Yes, this *is* my first actual lemon...and I count it as such even
though everyone had their clothes on throughout.  ("Making love" is not
necessarily the same as "having sex", after all.)