LOL That's cause porn is a form of "competition" which comes (no pun intended) at cost much lower than themselves. The same reason women generally don't like legalized prostitution... if sex is a marketable activity, then what most women offer is priced at what economists would call a "luxury good" (high priced).
From a purely economic perspective....In today's society, a normal relationship is a rich man's game. The costs or potential costs are very high and the benefits have not improved at all.
Porn and brothels are priced at a level below the benefits offered; which means that from an economic perspective, a rational man would use these things rather than a normal relationship UNLESS the woman in the normal relationship was offering something in the way of greater benefit.
Greater benefit is usually associated with smarter, higher class women. The problem many women haven't faced yet is that sex and relationships are a marketable good. If they want to get customers then they have to improve their offerings OR they have to as a group create a monopoly situation on sex and relationships. The monopoly option isn't possible...men are turning to porn, gay relationships (in many cases, that is a choice...their may be a predilection, but still there is a choice element to it), and to brothels.
By the way, the same need to improve offerings applies to men, but in many cases women will accept scumbags as boyfriends when they should hold out for something better. As long as women date scumbags, then men will not feel the need to improve on a widespread basis.