Thanks for all the positive feedback. I'm glad it's not only me that enjoy this 
In this next part Sarah explore a new and even more cruel side of herself. Starts a bit slow.
Me and Sarah were both quite busy with school and friends the next eleven days. It was of course frustrating to be denied orgasm, and it really didn't seem to get any easier after all the training of being denied. Sarah didn't care about that. Whining to her and begging for release was hopeless, and would only make matters worse for me. Despite being busy, she of course made sure I kept my private parts clean. Letting me unlock the chastity and clean myself, under her supervision. And she watched as I had my scheduled hour of porn during the week, which was still humiliating to do while she watched me. On a few occasions, when she came home from a long day at school and with friends, she fucked me hard with her strap-on, on my bed. Her instructions were always the same when she was in that mood; “Your bed now! Get in doggy position and present your whore-ass”. She fucked me hard and just left me there, which made me feel so used. But I loved it, and I think she knew that. She was hanging out a lot with a new woman, Ann. She was older than me and Sarah. About 25 years old, I think. Ann was studying the same as Sarah, but was a few years ahead and helped Sarah out a bit with school tasks. Sarah was doing very well with school now. Compared to before we moved here. She had found something she enjoyed, and it made it easier for her to be a good student.
I was following the same rules as usual. Always bra and panties, but allowed to wear boy clothes whenever I went outside or we had guests over. However, on friday, which was almost two weeks after Denise had left and soon time for my orgasm, Sarah told me to wear boxers to school and not to change when I got back home. When I got home after school, I did all the chores and had dinner ready when Sarah came home. She told me to sit with her by the table as we had dinner.
Sarah: So you know, Ann is coming over later.
Me: Yes, Mistress. Do you want me to have anything ready?
Sarah: Well it's just that. I want you to be a boy today. Wear only man-clothes and act completely as if you are not owned by me. Just like we are normal roommates.
Me: Oh, OK. So like when we normally have guests over.
Sarah: Yes, but don't be so nice, quickly doing anything I ask. I know they just think you are like supernice or something. But be more like a normal dude today.
Me: Yes, I will do that.
I was unsure if I had been doing this wrong every time we had guests over, of if it was something special happening today. We were quite and ate for a bit, before Sarah continued our conversation.
Sarah: So do you think Ann is hot?
Me: Yes, she is sexy.
Sarah: You know, her boyfriend is a complete asshole.
Me: Oh, how come? The cheating part? Is it true?
Sarah: Well, he don't seem to give any effort in their relationship. And yes, I think he is totally cheating. Even though she is sexy as fuck. What is wrong we you guys?
Me: I don't know. Some guys are assholes, I guess.
Sarah: Yes. Some of you.
Sarah had told me a bit about the Ann-situation. She had tried to break up with her boyfriend a few times because it wasn't working out, and he didn't seem to care about the relationship. Yet, every time he managed to talk her out of it, so they were still together. Ann was very sexy. Not like a babe, but more natural look. An innocently looking face, but with a smile that made me think she could be kinky. I'd had a few naughty fantasies about her. Sarah began to talk about Ann and her BF.
Sarah: He don't give a shit about her unless she wants to break up with him. Then suddenly he do whatever it takes to talk her out of it.
Me: Yes, I understand. It's not a good situation.
Sarah: And in the bedroom he give no effort. They occasionally have vanilla sex. Even though she has brought up that they maybe should spice things up a bit.
Me: You think he sleeps around and therefore don't need to have sex with her that much?
Sarah: How else would he manage to stay away from her sexy body?
Me: Yes it seems strange.
We stopped talking for a bit. I still didn't understand why I was told to wear boy underwear. We usually had dessert on Fridays, and today was no different. I cleaned up and brought it out on the table.
Sarah: Oh, this looks good.
Me: I hope it is.
Sarah: I've talked a lot with Ann about kinks and stuff like that lately. She is a very kinky woman.
Me: Oh, how so?
Sarah: Haha, so interested you are now..
Me: I was earlier too!
Sarah: Haha, sure. Well, she is very submissive. Seems very into being a slut, degraded and forced to do humiliating things. I really like it.
Me: Oh, that sounds very hot. Have you two done anything?
Sarah: Haha, nope. Nothing yet. She agreed to try a bit of play today, and I'm curious about maybe blackmailing her. I peaked through her phone when she wasn't looking and noticed a few sites she had been on about that sort of thing.
Me: Omg, that is so cruel!
Sarah: Why? She is into it, I think. Hehe. And I'm very curious about it now.
Me: And how do you plan on doing that?
Sarah: Oh, right. Good news for you. I want you to help me.
Sarah told me all about her plan. We would just watch a movie together, all three of us. And after that she would play with Ann in my Sarah's bedroom. Making her horny, and hopefully submissive and obedient. Sarah told me about how I would get involved. My chastity was removed before Ann arrived.