Dosukebe Bitch na ○gakusei no Sex Zanmai Nanokakan Sensou ~Homeless kara Himitsu Kichi o Torimodose!~

Summer vacation was ending in 7 days.
Four friends - Kenta, Gaku, Maki and Sasara - went out to their "secret hangout"
in a fenced-off area forbidden to trespassers. The boys left the girls to do their usual
antics, trying to set off bottle rockets out of view.

While the girls played alone, two homeless men invaded their space.
"Nice place to sleep," one muttered. "Mnahgh," said the other.
Maki immediately unleashed her fiery spirit: "Hey, get out of here!
This is our secret hangout!" Sasara was more timid:
"D... don't come in here..."

One of the homeless men leered. "Sex match," he said flatly.
(What was that suppose to mean?) Whoever won would had to
leave the area. Pinky promise.

[From DLsite English]