He does indeed allow it, and Erza gets rough!

"Okay, go ahead," Natsu told Erza. "And go ahead and make it sexy," he added.

"Oh, it will be," she assured him, getting up, and Requipping into a tight black leotard that left her thighs and arms bare, along with a fair bit of her cleavage. There was a zipper right over her crotch, and the outfit came with shin high black heeled boots, and a spiked choker, as well as rope in her hands.

"Oh, that's sexy," Lucy purred. "Do whatever you like to me, Mistress," she added in a sultry tone as Erza's demeanor changed.

"Oh, I will..." she smirked, now looking outright devious. "Just to be sure though, I'm giving you a safe word, "Red" will do..."

Lucy actually laughed at that. "Mistress Erza, I'm a Sex Genie! The only safe word I need is Master telling me to stop!"

"Fair enough, but remember it anyway, just in case," Erza grinned, before Requipping a rope into her hands, one enchanted to tie Lucy up auromatically. Lucy yelped, before grinning as the rope moved to tie a Shibari knot around Lucy's body, bringing her hands behind her back.

"Oh yeah! This is hot!" Lucy giggled, before Erza grabbed her hair, and forcibly pulled her up to kiss her. Lucy whimpered and kissed Erza right back.

"...isn't that a bit much?" Lisanna asked Natsu, before leaning down to give his cock a lick.

Natsu shrugged, before moaning with the sensation of Lisanna's tongue caressing his manhood. "I don't see Lucy being genuinely distressed, let them have their fun. If Lucy wants to use the safe word, she can," he panted.