From what you've described, the images that your dp has downloaded, albeit nauseating to many, are not illegal.
If the 'real life rape' is, in fact, images of rape being perpetrated on a non-consenting adult any criminal proceedings that may ensue will be confined to the perpetrator(s) of the act, presupposing that he or they can be identified and located. However, it should be borne in mind that many images that fall within this particular category of adult porn are carefully staged complete with shaky handheld camera to provide an 'amateur' look for the further titilation of the paying punter viewer.
With regard to the images of German 'teen'' fisting, although there is provision within German law to protect minors under the age of 18 from coercion by those over 21, the age of consent in Germany is 14 years. IME, professional (I use the term ironically) German pornographers do not seek to transgress the law of their country of residence.
Reporting these images to the police is therefore highly unlikely to produce an outcome that would satisfy some responders on this thread and, should you decide to act on any of the misguided advice given and approach your local constabulary, it is probable that all you will succeed in doing is alienating yourself further from your dp's family/friends and it will be a matter of conjecture as to whether your relationship with the father of your dc can be sustained in the short or longer term.
Viewing 'vanilla' porn can, for some, be a slippery slope to hardcore imagery and to rl sex crimes but the fact that he has viewed imagery that is particularly distasteful to you does not necessarily mean that he will attempt to enact any of these scenarios in rl or that he poses a danger to your own, or other, dc.
In the absence of a considerable stash of related or similar material, it may be that the explanation that he gave is the truth in that he did allow curiousity to get the better of him. If so, there is no reason to suppose that he found the material sexually stimulating or that he used it to get his rocks off.
As a society we are so inured to watching acts of violence and other depravity being enacted on big and small screens, both in fiction and in factual media accounts of atrocity, that the true cost in terms of psychological damage caused by pornographic imagery is overlooked by many.
IMO every voyeur of pornographic images should understand that what they are viewing remotely, so to speak, are acts that have taken place in real life and that the actors are sons, daughters, sisters, brothers, etc, who, in many cases, have been abused, ill-used, and sexually exploited from an early age.
As you have said that he has 'joined' 2 dating site, I'm less inclined to believe his contention that he clicked on these out of curiousity and IMO his reasons for signing up need to be explored in depth.
It may be, as Charbon has suggested, that he is seeking opportunity to engage in a tit for tat exercise to get back at you, as it were, for your admtted infidelity but, if he is harbouring thoughts of this nature, I would have thought he could accomplish his 'revenge' in rl and therefore doesn't need to engage with strangers on the net.
What appears to be abundantly clear from your post is that you haven't as yet discussed these issues in depth with your dp.
Therefore, IMO you are best advised to seek individual counselling for your issues of low self-esteem and, if you are unable to engage in constructive and productive dialogue with him, the pair of you should give consideration to engaging in joint counselling as exchangng views in the presence of a neutral third pary may help you facilitate a better undersanding of each other and of your individual learned and perceived opinions of certain topics.
It could be that following any such frank and full disclosure you may decide to go your separate ways but, given that his is the father of your dcs, it will be infinitely preferable to part on good terms rather than in a knee-jerk confusion of emotive recrimination and possible misunderstanding.
With reference to your current pg, this came about not because he whinged/begged for sex to which you eventually consented but because neither of you used contraception and I would suggest you address this particular issue after you have given birth.
As for bleaching your brain free of the images you've seen, time will enable you to file them under the general heading of items you wish you hadn't seen, read, or otherwise been exposed to. All sentient beings have a selection of these unwelcome items tucked away in the recesses of their consciousnesses - my own such files are extensive and occuply numerous securely locked filing cabinets unfortunately some persistent images escape to haunt me from time to time
I sincerely hope that your mind will be eased by the realisation that, on the basis of what you have said, there is no cause for hysteria or for making a drama out of a crisis, and that there is no probem that cannot be resolved one way or another sooner or later.
FTR, the above is in no way an apologia for your dp's behaviour nor should it be seen as any indication of my personal opinion of pornography