Women who have been sexually assaulted by partners/close rtelatives are less likely to report or even believe that a sexual offence has been commiteed - this is due to a number of factors, but one of which is a rationalisation that they do not want to catagorise people they love/depend upon as rapists.
The rape laws in this country have not been changed for over 100 years - they were written when the Victorian midset about women being 'coy' and asexual was still the common consensus.
As for why men rape - it is simple really - they rape to gain reproductive advantage over their rivals. If they cannot access a female by legitimate means, they do so by other means.
The debate at the minute is about whether the male propensity to rape is a direct adaptation or a bi-product of other adaptations, such as high levels of androgens such as testosterone which are present if a male is to win in a fight for females, be it mano a mano or in gathering resourses to attract the best females to him.
The most obvious evidence against the direct adaption theory is that human males to not have any special built tools to assist rape, as other species do. This isn't concusive however, as Orangutangs do seem to have an adaptive rape behaivour and tehy have no special built tools either.
For me, it just seems a bit like a cheater strategy. Rape is known to be an opportunistic crime which is even more likely to occur if the chances of being punished are low - as they are even in the West.
There are taboo's on rape in almost every culture (and rape occurs in every culture too, contrary to popular belief) - mostly from kin groups and communities. When communities become atomised, and women (especially young women) move away from home, they are known to be in a very vulnerable group.