SOLVED HR. Heroine has zero knowledge about an intimate part of marriage. Not even a vague idea.

This might be a difficult book to find because I remember only one scene.

H & h are either forced to get married or have a marriage of convenience.

It's their wedding night (I think they are rushed into consummation by someone and hero is not happy about it - but I'm not 100% sure). He is in a bad mood so heroine assumes he's scared and nervous about bedding her. She undresses to her shift, lies in the bed and with a gentle smile invites him to join her, reassuring him that there's nothing to be afraid of. When he does, she wraps her arms around him and stills in a hug. He's perplexed by her behavior and asks what she's doing. She answers him that they are consummating their marriage... because she thinks that lying with her husband means literally that... lying in bed with her husband and nothing else. She's absolutely oblivious to what happens between husband and wife.

I almost sure it's medieval era.