Shaun is a tricky one. You will need to stumble upon him in the wee hours of Saturday to Sunday night, when he returns from the Night Club. He'll be in the kitchen, quenching his thirst. I don't remember exactly how many conversations you need to have with him about the Night Club, but after some time he will ask you on a Wednesday morning if you want to go clubbing with him the coming weekend. If you agree, make sure Becca is wearing a little black dress or equivalent on Saturday evening and is at home. When in the Night Club, you'll need to use bathroom twice for Shaun to finally follow Becca there and offer his BBC for her to suck. Then return home and you can have sex. Pretty convoluted, isn't it?
The medium dildo acquisition is even less obvious. First and most of all, Becca cannot be a virgin, she has to accept a live human penis in her pussy at least once for this (and many other events) to trigger. Small dildo in pussy will not do, Becca's hymen needs a real cock to yield. It will be Abby who will one day 'challenge' Becca to go to the Sex shop, buy a medium size monster of a dildo and make a video of herself playing with it, because you know, regardless of how much porn she's watched, live webcam shows performed and how many sex partners she's had or how many bottles of lube she's bought in that Sex shop, she wouldn't buy a dildo one size up by herself. Even the most sexually liberated women need a corrupting agent, I guess.