Actually there is such a thing as presumed consent. If a court is presented with evidence that two people had sex together, and no evidence about consent or lack of it, the court will presume that both parties consented. This comes up in the family law context, where evidence of sexual activity is often necessary to, for instance, show that there is good reason to order paternity testing, or to prove adultery.

If a man seeking divorce on the grounds of adultery can show that his wife had sex with some other man, the court will presume she did so willingly unless presented with evidence to the contrary (if a woman is the victim of rape, the act of intercourse involved is not considered adultery).