Add on to that - Foyet is all about power and control .... he would want Hotch struggling to prevent a rape .... why stab him 7 or 8 times, then rape him? He's more likely to have stabbed him once to weaken him or whatever, then raped him, then stabbed him again .... he'd want Hotch to struggle and by the time Foyet got around to mentioning knives and sex, Hotch was in no condition to fight .... no satisfaction in that scenario.