'Rapists commit rape because they have something wrong with them/their minds or personalities'
Yes, it'd be nice to think so, wouldn't it? Then we could just profile them, spot them all, and get rid of rape.
Sadly, again - it doesn't work.
There are many men who would never rape, no matter what the circumstances. However, there are also many men who seem like perfectly ordinary guys, but will rape if they think they can, or if they've been persuaded this kind of rape isn't really wrong, or whatever.
If there were some kind of mental flaw, or personality flaw, that forced men to rape, there would be a constant number of rapists in all societies. The passing of laws, such as that criminalizing rape in marriage, would have no effect on wider attitudes. Yet they clearly do (even if it's much less effect than we'd all like).
I wish people would stop making this claim, it is incredibly damaging and two minutes' thought would tell you it's not that simple.