What if a woman had sex with a man and then the woman claims he raped her? Would he go to jail?

So this is a question I am just wondering, I know it’s kind of silly but anyways. Lets say a boyfriend and girlfriend or a husband and wife had sex, they both including the woman willingly had sex but then the woman went to the police and claimed that the man she had sex with raped her (maybe she want to sue or stuff like that) and she didn’t want to have sex with him. What would happen in this case? What determines if it was a rape or not? How can the man show that the woman is lying?