Thank you so much for writing me back. I wasn't able to be online yesterday, so I am going to try to provide an answer to your responses right now.
As you know, I'm sure, from the beginning of time, literature has always included "taboo" references, such as the "Oedipus Complex in Hamlet" where sons grew up to be jealous of their fathers and fall in love with their mothers. Socially, however, this was not really ever acceptable. In fact, there are laws on the books in every state to protect children (minor children) from sexual abuse from parents, other family members, and friends of family. So just keeping that in mind, I know you know what you are asking about is not only taboo but considered "wrong" by many people in society, including those in law enforcement.
Now, enough said, you both are consenting adults. And while I won't impose my value system, I do want to caution you against the can of worms you undoubtedly will be opening should you decide to tell your mother about your desires. You indicated that she had found porn, but I am still not seeing the connection to her?
Now, your other statement, "I still find the idea of sex with her highly arousing." Is it because doing so is so taboo in society? Or is it that you find her body so beautiful and sexual that you want that physical pleasure? Here is the crux, I think. When people are very sexually attracted to one another but do not want the emotional connection of a romantic relationship, then the people usually engage in a one-night stand or at least a very short, physical-only affair. Then the physical novelty wears off and each person moves on.
The problem in your case is that you really cannot "move on" once you have a sexual relationship. This sexual relationship will always be there in the future, whether you get married and have children and she comes to see the grand children or not. How would you explain this to future girlfriends? Would you? Would you try to hide it? If you would try to hide it, then why? Are you ashamed?
I would like to talk more about this with you. At this point, hopefully I have given you some things to think about. Again, please understand I never ever judge customers/clients. I just try to better understand, think with research in mind and also compassion, and give the best recommendations possible. So let me know your thoughts and if you can accept my responses so far. If not, I can either opt out and let another expert help you or I can try another pathway. :)