I don't mind my boyfriend watching porn. I would only have a problem if it affected our sex life, he was hiding it or lying, he was cheating, or if it was an addiction. However we actually sometimes watch together as part of the foreplay and the only other time he does it is when AF has come and I'm not feeling it. We've talked about it and we're both comfortable with it.

ps i'm not bashing anyone who doesn't allow/like their SO watching porn. every relationship is different and it should be your decision.