Against the wall.

"Darkness, go ahead and back up to the wall," he ordered her.

"Yes Kazuma!" she replied excitedly, backing up until she was against the wall. Kazuma walked up to her, caressing her abdomen, and eliciting a coo from her before he kissed her, sliding his hands up her torso to cup her boobs.

Megumin meanwhile quickly got up onto the bed, and moved herself towards the head of it, which was against the wall Darkness was on, so she could get a good view from the side, a bit of drool escaping her mouth before she wiped it up.

Darkness proceeded to lift a leg up to wrap around Kazuma's waist so that her pussy was exposed, with his erection pressing right up against it, and he wasted no time thrusting into her. She gasped out and moaned in joy as he began to fuck her slowly, each thrust harder than the last.

"Oh wow, that's hot," Megumin cooed as she watched this show.

"Indeed it is," Kazuma agreed as Darkness wrapped her arms around his neck, and her other leg around his waist. He sped up his thrusts into Darkness as he fully supported her weight with the help of the wall. Once he was outright pounding into Darkness, Megumin caught herself reaching down between her legs to finger herself, before quickly pulling her hand away, not comfortable with masturbating in front of others yet.

Kazuma of course noticed and made a mental note as he mentally grinned that he had found a line for Megumin. Darkness cried out in ecstasy as he got her off, her pussy spasming on his dick as he also came, filling her to the brim with his baby batter. Well, sterile baby batter, since he wasn't planning to impregnate her, but still.