Shame. In our society at least, it's inconceivable in mainstream thinking that the male can be the victim; it always has to be the female. It's bad enough when a female is sexually assaulted, God knows, but, in my opinion, it's almost worse when the male is the victim. My colleagues may disagree with me on this, and I hope all those with diffeering opinions chime in here. It's an incredibly important.
Guilt. Because they somehow believe that they should or could have avoided the assault, or the mistaken belief that they provoked it themselves. Hence the reason that they usually don't report it, or fear that the pervert who assaulted them will harm their loved ones.
Shame. Very complex question. Usually at least three feelings; guilt , being ashamed and shame. Shame is a feeling of confusion that what is going on is not good. Guilt , being ashamed are feelings that we will be punished for it. How we know that it is wrong at such an early age is not an easy question to answer. Shame is necessary , guilt and embarrassment are not. We learn the last two.