Why do feminists think that a man should know that a [woman] doesn’t want to have sex if she doesn’t say no or indicate not wanting it ? How is that rape?

Repeat after me: specificity and inclination.

Got it? Good. Let’s apply them to other situations.

A man at a restaurant sees a woman dining with her friends. They’re all having pie. Thinking she must like dessert, he walks over to her with his chocolate cake and starts shoving it in her mouth.

She never said “no” to having a stranger’s dessert. But she was already eating a different dessert with friends, so why would he assume she wanted his?

A woman is the lead guitarist in her funk band. She’s getting down and she and the crowd are feeling it. Then, unannounced, a man with a fiddle jumps up on stage from the audience and starts playing the country music tune “The Devil Went Down To Georgia” right next to her.

Again, she never said “no” to playing music with others. But why would a country music fiddler assume he could, unsolicited, start playing HIS music with a funk guitarist IN A BAND people paid to watch perform in a DIFFERENT style and genre?

A man sees a woman dancing on the dance floor. He’s incredibly turned on by her dance moves. He gets up, heads toward her, starts gyrating and thrusting his pelvis at her, and starts miming fake-slapping her ass. She turns away from him and dances in another direction.

No, she didn’t say “no” to dancing with him. However, she never indicated (a) what type of dancing she likes doing with a partner or (b) whether or not she wanted to partner up in this dancing instance.

And if this man, in all these scenarios, isn’t neurotypical and/or has trouble reading social cues, he should make arrangements to figure out context and intent before acting (or have it figured out for him in case he isn’t capable of it).

Now let’s apply specificity and inclination to the scenario you’ve posed.

Without knowing anything about her erotic preferences, inclinations, desires, intent, partnership status, personality, and a whole list of other factors, he simply cannot know what she wants sexually. And even if he knows all of those things about a woman, she might never say “yes” to him under any circumstances (or she might withdraw her “yes” for future interactions even after freely giving her “yes” to him in the past).

Her not saying “no” doesn’t just fail to meet basic standards of permission. Not saying “no” is actually a few meters underground and he’s going to have to excavate a tunnel from the cave he’s in to even reach where consent might or might not be waiting for him.