That is not to say that animal lust is not involved. Not a rape, but it's first cousin, murder. To return to Clary, if Lovelace wanted sex, he could go elsewhere. So it's more than just animal desire. It also has to do with an obsession with one person, here Lovelace with Clarissa.
Reciprocity is also part of a relationship, and each relationship is in various ways unique.
Finally the ambiguity of the situation comes clear if one thinks that Richardson has Lovelace drug, tie down, and humiliate Clary so we will be sure to define the final scene as rape. The issue was therefore not clear in the 18th century either. Also I have sometimes wondered if the people who say they prefer Lovelace would not define the events of June 13th as rape. One could cite the modern instance of Wm Kennedy Smith where the jury said he was not guilty of rape. That Clarissa was Drugged and Held Down Home Contact Ellen Moody.