Point blank: There's only so often a man can, uh, "go," both in the literal and figureative sense. If you're retreating to the computer at night (or when she's not around) every time you're feeling frisky, it can ultimately mean less sex for her. 

Most women are understanding if you're occasionally not in the mood, but if your reasoning involves anything without her, she'll undoubtedly be taken aback, if not offended. It's not that she's upset by you watching pornography period (though some women may be), it's more that you're opting to connect to the Internet instead of with her.