She wants to "tire you out" first

You let out a muffled yelp as you feel something large and cold pressing against your asshole and correctly assume it's a glass buttplug. Again your anus protests but due to the lube and its shape, you feel slowly slide in. You cry out weakly as your asshole stretches to accommodate the smooth, cool bulb. As soon as the widest part of the the bulb was inside, your anus suddenly swallowed the rest on it's own, your sphincter closing tightly around the neck of the plug and causing an echo of pleasure in your other hole.

"Wow, I can't believe the bigger one went in so easily, good job sis!"

The plug didn't exactly hurt, but it was definitely uncomfortable, and despite your body's best efforts to expel it, it remained tightly sealed by your own sphincter muscles. Since you were still bound, it wasn't going anywhere without Gwen's help, and since you were gagged, you can't even ask. You wished your cunt would stop throbbing in approval of your own helplessness as she flips you over. In her hand you spot a small vibrator.

"I figured that since those pelvic muscles are so strong, I'd have to tire them out a little first..."

Your eyes widen as it dawns on you what she plans to do. You try and escape her, backing away as best you can while shaking your head wildly, but she easily pins your helpless form to the bed, her free hand holding you down by the neck as she looks directly into your eyes.

"You can't run away now, this is punishment, remember?" She says, eyes full of sadistic lust as the vibrator buzzes to life. She decides to tease you, starting at the top of your body. Even rolling it across your collar bone was enough to make your pussy clench in response, as if your whole body was an erogenous zone. You mewl as she rolls it across your nipples. Your breath quickens as she rolls it past your navel and onto your pubic area, and without warning she presses it to your clit, making you cum explosively. You feel your ass clench around the unyielding plug, causing a secondary wave of pleasure. Gwen holds the vibrator to your clit, sending you into a second orgasm almost immediately. Your hips spasm and shake wildly as the sensations overwhelm your body, but Gwen doesn't let up. Because of how she's laying on top of you, you can't even close your legs and she's free to **** as much pleasure out of you as she wants. After your fourth orgasm she pulls the vibrator away and admires your flushed, tear streaked face.

"You cum so beautifully..." she said. Her own face was flushed and biting her lower lip. Through your tears you notice her almost painful-looking erection. If she changed her mind and decided your cunt was better it would take less than a minute to seal your fate. To your relief she rolls you over and pulls on the buttplug. Your asshole still tried to hold on to it, but after Gwen's **** your sphincter muscles were exhausted and it came out with surprising ease.

"Perfect" she lines up her cock with your now looser asshole.