Rape is non-consensual sex.
There's no "just an unwanted sex act". The integrity of the body is a fundamental human right and another person may only interact with that without your consent. Period. Anything else is an act of hostility.
And by permission I do not mean waving away of rights before the act, but a continuously expressed consent. So if at any point you are no longer ok with it, the other person has to accept that and would commit a crime if they proceed.
The problem is, while you can formulate that in no uncertain terms, the practical enforcement of that is a nightmare. There are usually only two people involved in that, meaning you've no witnesses and two directly contradicting accounts with little chance to verify either and the juridical principle of in dubio pro reo (when it doubt for the accused) often shields the perpetrator (or the falsely accused, it's not that easy). So from the law enforcement perspective the focus is often on penetration, bruises and physical violence, because that is the kind of things the forensics team could verify.
While the juridical part has to prove the "guilt of the defendant". Which is as hard of a task as it sounds and even in the case of such marks of force, a defendant could argue with "they liked it rough" and "they consented at the time" and you'd have a hard time proofing that wrong.
So in the worst case scenario in the face of a lack of demonstrable dissent, it's not rape, but not because it isn't rape, but just because the court can't proof it.
However regardless of whether a court convicts the perpetrator or whether the perpetrator is aware of their wrongdoing, the action itself nonetheless can have a traumatizing effect on the survivor of it. Which may or may not correlate with the amount of force the perpetrator has used. So just because an act of rape didn't leave bruises and visual markers of any sort or involve penetration doesn't mean that the experience is any less severe and traumatizing.
So from the perspective of the survivor any action that can cause that effect could legitimately be called rape. From the perspective of law enforcement it's only what can be proven and from the perspective of the accused it's only what happened in malicious intent.
Not to mention that the specific legal definitions may vary from country to country and would be better suited for a law stackexchange.