Hey look a tinfoil hat
(add to ignore for sanity sake)
Ah, I hate tinfoil hats and the usual conspiracy theories just as much as the next guy, but this here isn't tinfoil but can easily happen.
If you are accused of something morally untoward (especially concerning sex) and somewhat known it will go out there. The more you are known the more major the headlines. If then nothing comes from it, especially if the case is thrown out early enough, the reporting on your exonerance will be comparatively minor. And even if it were major, many people still like to think you're guilty or at least shady - "where there is smoke there is fire" style. Some bad stuff sticks, and especially if you're not some employee but reliant on other people to trust you personally as a good business partner, this may severely hamper your career.
Over here we have two TV personalities who have been accused and completely won their cases - not even for lack of proof because "he said she said" but because it could more or less be proven that nothing like the accusations happened - one even wasn't in the country he allegedly did stuff. Yet, one of them is relegated to minor stints on regional TV instead of prime time national TV, the other doesn't get anything anymore at all. Too risky to hire them, almost guaranteed shitstorm.
Winning a court case can save your legal reputation, but since the thoughts of people cannot be forced, you may still take heavy damage.
Those photos, at best, are only good for one thing at best, blackmailing, if the MC is a beta. Pretty sure the MC is not the "legal" father in any sort of document, the triplets are all +18 canon, that kiss was consented, and worst of all, those pictures were took illegally in a total breach of privacy. So if Glenn goes to "court" for whatever reason, in a logic real world scenario (which Radiant already proven is not going for) he might end up paying a big sum to the MC while being sued... and gettig nothing from the pics. Besides, the MC at that point can say how Glenn's been harassring him and maybe add a couple years in jail. Realistically, Glenn is a loser with nothing on the MC.
You have a good point about not being the legal father and being 18+. However, being "beta" has nothing to do with it. Quite the opposite, if you're known as some kind of (real or self declared) "alpha" who may "grab them by the pussy" so to speak, allegations will stick way better in the above scenario. Not in court but in the public opinion. Of course the asshole risks a lot himself - but don't think he could pay a lot of money. He may even get hit worse than MC due to this stuff if put out in public but the thing about blackmail is - even if you're right, you may still be pulled down.
First I do think that Glenn really has very little here, but in general it just isn't that easy.
Logically most blackmail shouldn't work in the first place - nude pics? Well, we know that women have breasts, men have penises, and everybody touches their own from time to time. The crime of blackmail is way worse than almost anything you could have done but that is often of no use for you. Cop goes down in prison, okay, but MC may have to shut down his company because no girls with higher profiles want to work for him anymore - "You are dead and I only lost an arm and two legs" is only a very limited way of winning.