Tell her to cum up your ass.

"Hey Sandra, not going to happen. It's too big!" You demonstrate by trying to fit her now wider cock head inside of your mouth. You can't even get up to the mushroom head!

"Oh, you're right sweetie. You will need lots of practice to deepthroat this," she punctuates her comment by slapping your face with it a few times. "But you know if you aren't going to suck me, I need to ejaculate someplace where I won't make a mess..." She looks a bit nervous.

"It's OK to cum up my ass, Sandra. Just take it easy this time, I'm sore from when you got wider." You lay down on your back, and you guide her wider head to your cum-slick butt. Her cock twitches as you rub it around your crack to lube up her head. Then you position it at your asshole and pull her into you a bit. It is more sore than you expected, and it hurts a lot, but you know she won't last long. She fits only about the same length as she did last time, and then gently slides in and out of you. You can't wait for it to be over. Without even picking up her pace, or giving warning, you feel hot liquid jetting inside of you with great ****. She stops moving her cock in you as she cums, so you actually enjoy your first time getting cummed up the butt. She cums a lot, and you love the feeling of being filled by her hot semen. Sooner than you would have liked, she comes down from her orgasm, and pulls out. Your sore hole closes tight after it leaves, trapping all the cum inside of you.

"No mess," Sandra says with a grin.