For some Japanese women at least, not only is having a boyfriend fall for a 2D character considered cheating, it is regarded as being worse than having an affair with a real woman or going with a prostitute.
Several Japanese ladies in their thirties were interviewed about their thoughts on the matter:
“If my boyfriend fell for a 2D girl it would be absolutely gross.”
“If he went with 2D I’d really feel there’s a problem with me, as a real woman.”
“I’d be totally shocked. I think I’d think the problem was with me and I was lacking something. It’s just like a man with a girlfriend pursuing a fake romance with a hostess or going with a prostitute, isn’t it?”
“If he is going to fall for someone else, at least make it an idol. You just can’t compete against a 2D character.”
Possibly deep seated insecurities about competing with 2D girls are one of the reasons Japanese women despise otaku so completely.