Thank you everyone for your suggestions.
perfumeditsawonderfullife and Nyx - it would seem, from my dad's behaviour, that he either wants to be found out, or doesn't think that others are capable of putting two and two together/ thinking or wondering about the things they find on a computer - i.e., if we see a minimised window, it's as though he thinks we're never going to click on it or read its title - or that we are but we're not going to think/ deduce anything? Like we don't have minds of our own? What I don't understand is, if he's not covering his tracks, why is he also so secretive? Why does he minimise windows when we walk past? I'm not a twin - :) - but I envy the person you dated, Nyx, because with the internet, it's like people have so much more capacity to indulge whatever they want to, and they don't necessarily keep it private. I do think my mum seems relatively content and has learnt to cope with the way my dad behaves.
cabbageroses - it is very possible that they have agreements that I am not aware of, and of course that's none of my business if they do. But that still leaves the question of why he's so obsessive (i.e., looking at it during the day when the family is up and about) and careless (i.e., not hiding his tracks) with his seeking out of gay porn. It just seems odd. Regarding the whole term "abuse" - I think I've used it once in my posts, in terms of how he talks to my mum, but I REALLY don't want to be "the adult child living at home whining about her abusive father" - I don't think his treatment of ME or my sibling is so relevant. I think that while my sibling and I have one view of his behaviour, my mum would have another, and my dad would have another - he would probably say that he is a victim, that we all criticise him too much, that he's unfairly expected to be perfect. He has done some pretty awful things but I just don't want to wheel that out as I'm not sure it's relevant to my dilemma, which is purely about whether or not it is better to talk to my mum in the situation I'm in, or just stay quiet and keep "keeping my dad's secret", if that's what it is. Regarding later posts, and your suggestion that I just go up and talk to my father if I'm upset about this - can you contemplate that there are people in this world who would NOT react well to that? That not everyone would be capable of a mature and non-violent response to such an awkward and personal issue that creates so much vulnerability and embarrassment for him? If there is a risk of a violent response, is it worth it to still try to talk maturely to him about this, when I've had so many experiences of failure in the past trying to talk about much less sensitive topics? There are computers at uni, yes... and at my workplace.
Curiousnamechange - thank you for your input. It's wonderful that you love your dad so much. :) I don't think it would be a good idea for me and my sibling to "gang up on him", which is how he might see it. This thread has actually really helped me in having empathy for my dad, if he truly is a man who has been repressed by society's expectations and feels miserable as a result. I think he lashes out the way he does because it gets him what he wants - people get scared into silence and back off. I have tried so hard in the past to deal with issues in a sympathetic, respectful, non-confrontational way. For instance I have tried thanking him for the good things he does, I have tried to help him lose weight because he is unhealthily obese and has a lot of health problems as a result. He gets angry and says I am patronising him, when I'm not intending to. Yes, he has a history of being violent although he stopped, bar a few incidents, once we both reached high school. I think he would deny everything if I said anything to him about the porn. Regarding your later post about perceived abuse - I don't know. One vivid experience I have from when I was young (7 or 8 years old?) was him driving me about and I was crying in the back seat, and he was telling me that my behaviour could drive him to kill himself, and was describing this in detail - e.g., dropping me off, just disappearing. I was not a bad kid. Is that a normal parent pushed to the limits, or an abusive parent? I don't know. He never mentioned this afterwards or apologised. He has done other similar things and to be honest, behaved worse towards my sibling. By the way, thank you very much for your kind words and speaking in my defense! (If I tried to do that I would be here forever.) Generally, whether or not my father is "abusive", I agree with you when you say it doesn't really matter. It's not the central issue here.
MakeYerOwnDamnDinner - thank you so much. In terms of my own emotions about all this, I have a bit of a "put up and shut up" attitude which may make me seem a bit cold/ detached when I write. I really don't want to list everything that goes on in the house. I do agree with you that if I try to "strike up" any kind of discussion about this (and honestly, why on earth have I been put in this situation where this is something I have to consider?!?!) he could lash out. The only thing stopping him now, I assume, is that we're big enough to fight back and old enough to know how inappropriate that way of dealing with things is. You are VERY right that I should not feel responsible for maintaining this kind of "walking on eggshells status quo", which I, to some extent, do. I'm sure my sibling feels the same way. I do feel like a domestic abuse organisation might be going too far and my mum would think it's crazy/ unnecessary - it is not so clear cut when the person is not beating everyone regularly, etc.
EldritchCleavage - thanks for making that point. Also keep in mind my sibling is equally knowledgeable about what is going on. If that is what he is trying to do I don't really understand his behaviour. I really am erring towards thinking that he just thinks we're so stupid, we wouldn't notice his sloppiness/ carelessness. He can be very lazy - I described his daily routine in my first post - sleeping, eating, TV, computer, repeat - he doesn't even indicate anymore when he's driving. It is possible that he is just continuing to reduce the effort he puts into covering his tracks, because no matter how sloppy he gets, nobody says anything, thus reinforcing his behaviour?
tadpoles - thank you, yes disengaging helps a lot. His lack of discreetness is really odd. I have friends where a parent "came out" and it caused a big disruption and great distress to the family - but I genuinely believe that would be better than this, at least everything would be out in the open and people can move on.
VivaLeBeaver - thanks, it must have been terrible to have that weight on your shoulders about your mum's affair. I have a close friend whose parents are also struggling financially late in life due to splitting up, they probably can't retire until they're in their 70s. The mum in that couple regrets the split despite instigating it, and never re-partnered whereas the father did quite quickly. My mum actually contributes more financially than my dad, but she would probably struggle with some more practical aspects of living alone, plus the lack of company, having someone to holiday with, etc.
mathanxiety - you have a very good intuition regarding understanding the family dynamic/ predicting likely responses and behaviours of my father. And a very healthy attitude, and a wealth of knowledge, that I would do well to learn from.
hairyfairylights - thanks, I agree.
Slambang - excellent suggestion, thank you.
MadameDefarge - ha, in many ways your post is exactly what I think. "Nice to pets" = writing some good points, trying to be balanced. I don't want to paint my dad as some kind of hellish demon. He is just very difficult. I TOTALLY agree that "them's the breaks". Things could be better, they could be A LOT worse. That's why I don't want to dwell on myself. I just want to know what would be best for my mum.
dittany - thanks very much. I don't view myself as a "child" and don't think my dad has "forced" us to look at his gay porn. But he hasn't concealed it from us either - and he didn't protect us from it when we were children, with my sibling finding stuff on the computer. I think it's more a matter of a lack of boundaries/ respect/ consideration. I really don't want to see the stuff my dad has been looking at when I use the family computer.
I think it does need to publicly be said that I am not male - just to stop unnecessary speculation/ wasted time by other posters in writing responses based on that assumption. But if I keep trying to address every false assumption, insistence on believing things that aren't true and that I've said aren't true, etc., it would be pointless and a waste of time and it also would piece by piece reveal more and more about the specifics of my situation. I've received a lot of really good advice here, thank you. I won't just disappear so if anyone has any good advice/ perspectives I would be very interested in continuing to hear them.