This reminds me of my girlfriend. She's kind of insecure, has an eating disorder, daddy issues, etc. Kind of a drag sometimes. She spends way too much time worrying about her appearance and thinking theres something wrong, even though she's actually attractive without the makeup and could stand to gain some weight.
Sometimes when I'm really horny, I get the urge to mess with her. I would say stuff like her pussy is too loose for me to cum, so I need to fuck her in the ass. When it first started, I convinced her that she'd do it for me if she loved me. Then I kept promising I'd be gentle or I'd put in the tip but invariably it would end up being neither. Now she just gets a look of resignation on her face and she will actually turn over and get on her knees so I can lube her.
My cock is pretty average so its not like I tear her up but it looks pretty big compared to her skinny little ass. Anyways when I start I always give her compliments about how sexy she looks with my cock in her ass or how tight her ass is etc. Definitely not things she wants to hear, hehe. What really gets me hard is when I hear her grunting in discomfort. I will hold her by her hips so she can't squirm away and pound her hard, and she will grunt and breathe hard while trying to crawl away.
Afterwards I give her lots of attention, make her breakfast etc :)
