The whole point of blackmail is that if the girl goes to the police, then you're going to do / reveal something that she doesn't want you to. Just because the guy is going to be in prison eventually (IF and only if she proves her case with actual evidence, which isn't very likely, actually) doesn't mean he can't bring her down with him.
Also, no, you won't go to prison for sexual harassment, lmao. Let alone for making illicit photos. Those are administrative offenses in most countries, and hard to prove also. In fact, the vast majority of RAPE cases end up having not enough evidence to land the perpetrator in jail. Interesting fact - Chikan (google it) is common in Japan, and there's not much authorities can do with it. Especially considering that some women actually enjoy it and probably look for men like this on purpose.
What you're talking about is more unrealistic than actual hentai plots. Sexual crimes are extremely hard to prove, and women know it. On Tor there are entire sites with "real blackmail porn". If you threaten someone with something bad enough, then they'd rather have sex with you than go to the police and risk you doing the bad things you promised. And, unlike what the society makes you believe, sex under pressure isn't as much of a big deal to many women as they say. Many are ready to have sex with strangers for 100$, let alone for saving their marriage or keeping their job.