We get far too many questions like this, asking us what to do. I have to say that if you even need to question us about the correct course of action, then you really aren't thinking properly.
Being propositioned by younger men, even if he is your son-in-law, is, I'm sure, very flattering, and makes you feel young and attractive. It may even make you wish for those days before you were married, and perhaps had the freedom to satisfy your own sexual desires. Or perhaps your husband was the 1st partner you ever had, and you now wish that you had had some experience before settling down.
You are tempted to have sex with your daughter's husband. At the same time, you are still married, and it is obvious that you and your husband are still having sex. Is the sex with your husband so dull or disappointing that you are enlivening the whole thing by fantasising about your son-in-law? Fantasising about these things is harmless, and many people weave the most intricate and perverse fantasies imaginable, in order to make their sex life more exciting. You are considering taking things further and I have to point out that while I'm not the least bit interested in condemning your morals, which are your own business, I have to do my duty and warn you of the consequences of allowing your fantasy life to become a reality.
The world is full of people who are having extramarital affairs, swinging, threesomes, one night stands, and so on. The one thing that is a common thread is that these things start out as (at least in the mind of the perpetrator) a harmless little adventure. However, once started, where does it take you? The husband who starts having an affair with his secretary, the bored housewife who starts having an affair with a younger guy, the auntie that starts having sex with her nephew……… Choose the scenario that suits your own circumstances; none of them consider what the long-term outcome will be. To choose the 1st example: the husband who starts sleeping with his secretary. She might be young and pretty, perhaps a lot more so than his wife, but she has needs, ambitions, and is looking for love and partnership. So what is he going to do in the long term? Leave his wife? Breakoff the affair? It can't last forever, that's for sure. And it's the same with any other clandestine activity. If you start these things, you better have a clear idea of where they are headed, and what the end result is going to be.
If you give in to your own fantasies, and start having sex with your son-in-law, the consequences in the long term will hurt you, whether anyone else discovers what the 2 of you are doing or not. The most obvious consequences of getting caught are that you will destroy your own marriage, and that of your daughter. Not only that, but you will tear your family apart, and you will suffer recriminations from all sides. Of course, you may not get caught by anyone, but eventually, you will have to face the fact that sooner or later your sexual attraction will wane, and what might have been exciting for your son-in-law will eventually become an encumbrance that he doesn't want any more.
You could decide to do this as a one-off, but how can you guarantee that he won't come back for more? Particularly if he goes through a rocky period in his marriage, or his wife is heavily pregnant, or he's not getting sex for one reason or another. Do you want him turning up, with nothing more than an ambition to use you?
Having sex with your son-in-law is your decision, and no one else can make it for you. All I'm saying to you is: think carefully, and make your decision when you have weighed up all the possible consequences. If you are thinking clearly, you will make the decision that suits you best.