I am always amazed at the more prude part of our population who rallies against sex before marriage and then generally become the people who have multiple marriages, regrets, etc. The fact is thought that sex before marriage is based on your hormones and your urges-sex during marriage is an obligation-even when you both like it. There will always be that difference.
Of course it isnt relevant now, but I still believe in getting your sexual oates sewed while younger-college aged or 18-25 and I feel that way because 1-they are they years where you are less likely to be married-thus less likely to cause a major rift in a relationship and 2-you are at your best health wise.
At 40, you and your husband may well enjoy a healthy sex life, but the sex you are having isnt likely to be olympic quality or at the very least, be aware that your perspective partners will not be the same people you see in pornos or at a night club-your age will be about 10-15 years above theirs and your energy level may not be the same.
More importantly is the "rules" you mention. And not just your rules, but the ones which you will be playing into. Yes, as a woman, you can put on makeup and a slinky black dress, and I assure you finding a man of most any age willing to have anonymous sex with you wont be a hard thing to do unless you are in need of serious plastic surgery, but your husband wont be able to hide his age and most likely, the club scene wont favor his efforts-he will be forced to use the internet dating sites or worse, the adult scene (strip clubs, prostitutes, etc). The dangers of diseases, drugs, etc which come with this are not to be ignored.
And Prudie mentions the rules you made, and I must agree with her-how the heck do you plan to not fall in love with someone? How do you plan to not feel jealous? Feelings are well known for overcoming the challenge of logic. Yes, your husband is a wonderful man, but what if you meet this new guy and he is also a wonderful man? What if HE meets a wonderful woman?
As an adult you know that sex is fun, but it isnt a dominant aspect of most peoples lives once they have kids, careers, etc.
My advice is use outside THINGS to inspire yourselves. Yes, rent some porno, even go TOGETHER into a strip club and get motivated-men are visual-seeing another woman will be easily translated into passion for you.
Dont think that you can make up for a goody goody past. Your past is your past. Anything you do is for the moment or for the future.