It happens at random.
This is a pornographic survival simulation, going around collecting things is the point until you get a supply chain set up through the girls, but you don't have to once you get the girls to start doing it on their own, which you can do by having them follow while you collect, cook, or sleep to teach them how to do each. Then you just have to worry about dealing with them when they want sex or when they get sick or exhausted.
That said, once you do get them to the point of sex, they can and will say no at times when you ask them for it, but they will come to you pretty often if you're in range when they get horny enough to deal with it immediately instead of a different need. Otherwise they'll find a place and masturbate or another girl that is in range if you are out of range.
You can't, this is an optimization issue in the game itself that has never been fixed, even high end machines have lag.
