There can be many reasons. All of them can be understood and overcome.
First off, you’re not alone. Lots of men who have had unwanted or abusive sexual experiences struggle with shame.
Shame is about who you think you are. It involves feeling unworthy of respect from others, feeling like you deserve to be judged and criticized, and feeling embarrassed.
We could almost answer this question with another one: How could a man not feel shame after having sexual experiences that totally conflict with how society tells us males are told they’re supposed to be? We’re told:
In short, you’ve always gotten the message that what society deems as ‘real men’ don’t have the experiences you’ve had, so no wonder you feel ashamed about having them.
It’s also possible that the shame you feel, if it’s very intense and almost constant, has other sources, particularly shaming experiences with parents or others that left you feeling “less than” and unworthy of respect or care. Both shame and guilt are discussed in more detail in the Masculinity, Self-Esteem & Identity section of Topics.
Finally, and we can’t repeat this enough: You can overcome shame, even the most extreme shame.