Watching other having sex is not bad thing. Rape is bad, having sex with someone who doesn't want sex with you. That is bad. Beating wife, that's another bad thing. Cheating. Bad.
Having sex with yourself is not bad. It is completely normal, not to say that it is recommended in lack of partners.
Why would be bad watching porn with actors that are complete strangers.
You have sex for 2 reasons, one is to make kids, other is to enjoy in it.
Well, when you don't have partner, having sex with yourself is NOT BAD. It is natural, and useful.
Watching own sister in porn, yeah, that's bad, but my sister doesn't shoot porns. Watching someone else sister, hell yeah. After all, I've married someone's sister. Having sex with her. I had sex with someone else sister too, before I met her.