What do you mean by corn websites?
Pornography site in short.
It is a no-brainer. Those websites are frequented by people who are mostly men and over Eighteen years old already, many of them are single and do not have a family, so that means they could have some extra money or pocket change in their bank accounts which could be used to gamble.
Is there a research that found out that many of the people that watch porn are not married? I think married men also watch it. Also it will not be a surprise if women, including married women are watching it than men do but they will hide it than men.
I don't have the source right now of the information, but I have read before on the internet those who are most likely to sereach for pornography are young men, and young men tend to be married or not (depending on the social context of their country, I think). There are indeed also women who search for pornography, but they are not the targeted audience for casinos and bookies, as it is well known by all of us those who gamble are more likely to be men.
By the way, there is always some degree of stigma when comes to pornography, no doubt about it, though it is stronger when the person seeking for such content is a woman, so you may be right on women actually being into it but hiding it from their family members and their friends.
In anyway, before taking actions like these ones, the advertisement department of the casino or bookie already considered the impact of such strategy, in their eyes there is a net positive in doing this.