A law can have aims other than the obvious ones. To take an example, tobacco advertising is restricted not because tobacco advertising hurts a person't health directly but rather because it can encourage a person to smoke.
In the same way, given how difficult sexual assault is to prove beyond a reasonable doubt, fornication laws could serve as an effective tool in a prosecutor's toolkit. When we consider just how difficult fornication is to prove, a legal-aged man and a legal-aged woman having consensual sex in the privacy of his bedroom should not cause any problem since the police would never know about it in the first place. So it would mostly affect those engaging in more flagrant displays of fornication such as in public or in the case of rape for example where fornication could serve as an alternative crime to prove when the more serious one cannot be proved beyond a reasonable doubt. If you read the statistics, sexual coercion cases are sky-high in North America.
Yeah, great. Give the prosecutors another tool to oppress whomever they want. Because no prosecutor in Canadian history EVER misused his authority.