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Very interesting question Jacques,
During a period of enforced idleness, I applied for a job at a company which has both porn and non-porn arms as a junior programmer for a short period. As it turned out, I didn't get the job so it became a moot point.
The bottom line is that you must make two judgements IMHO; Firstly, can you reconcile working for a company which sells pornography and may, potentially, be involved in the creation and dissemination of malware with your personal principles? Secondly, do you think that it will prejudice future job applications? I think that this very much depends on the personality and beliefs of the person interviewing you for the next job, however I believe that it will follow you and may possibly have adverse effects on future applications.
You have already asked these questions, however I cannot give you any real answers to either of them, only share my own opinions.
On a personal level, I know the IT director of the aforesaid company and he assured me that they do not produce malware. I believe him. They also deal in adult material, involving only adults above the age of consent and require proof of age. I do not see this as a great evil, personally, however many will.
Good luck, whatever you may decide.
Elgon UPDATE: As per Jacques' post below, he's already turned the job down. This is what you get for skim reading with a beer in one paw. To be honest, I'd had a few beers and just kinda zeroed in on the word "porn".
It is better either to be silent, or to say things of more value than silence. Sooner throw a pearl at hazard than an idle or useless word; and do not say a little in many words, but a great deal in a few.
Pythagoras (582 BC - 507 BC)