_IceX_ wrote...
I don't think anyone what kind of guilt/regret that OP means. It's a weird feeling that also mention sometimes in the manga itself...
It was an ethical/moral thing basically. Some people feel guilty when they masturbate because it's in some religious respects considered a sin. Basically self pleasure is giving into temptation of the flesh which puts yourself before god (God must come before all in a christian sense). People of very religious backgrounds can have this conflict even in this day and age. Some people also see masturbation as the gateway to promiscuity aka once you start you'll only want more. Doesn't mean that it's right or logical, but again some religions/people are like that.
In hentai, some people get embarrassed moreso because they feel like such a pervert when they get off especially if you see some where the guy uses a girls picture or panties in doing so. Thoughts like "I could get caught" or "I'm such a pervert" come to their minds, but it all works out in the end. Takes time to learn to feel certain things are normal and not feel so self conscious.