Dawkin's 'logic' may be correct but his understanding of rape is flawed. Not surprising - as a man and presumably not a rapist, it is a subject that will not intrude into his nightmares, nor haunt him when he thinks of near misses or has to live in fear.
At WA I worked with many women who had been raped by a husband/partner or former husband/partner and learned that this was indeed horrendous RAPE, not 'merely' unwanted or forced sex but rather a show of control, a deliberate attempt to degrade, by using insider information, obtained during the relationship, to whatever was most abhorrent to her. When a woman is raped by a complete stranger it is just possible, though very difficult, to come to terms with the attack as having been random, a horrendous accident, she happened to be in the wrong place at the wrong time, she need feel no shame, she need never feel any connection with the rapist. Whereas for a woman raped by someone with whom she has been in a relationship the rape is exceedingly personal, she has been directly and specifically targeted. In addition she may not be believed by close family and friends or may find it impossible to seek their support and will have an ongoing relationship with her rapist (perhaps the father of her children) and his family and friends. (Imagine trying to tell your MIL what her much loved son has done or your brother/father's reaction when he learns what has happened.)
And NO - I am not trying to suggest that any 'type' of rape is worse than another. Only that a relationship rape has its own distinct, often unacknowledged, characteristics.