No this isn't a 40 something disease..........this is a normal sexual fantasy that many people experience and there is nothing wrong with you. Many people experience this regardless of how happy they are or how great their sex life is. There are many people who claim that if shared together that group sex or swinging can be a enhancement to the relationship..........just as there are many who claim it has destroyed their relationship. Add the threat of STDs and your own feelings and it is a hard subject to talk about with your partner.
The problem is you are still acting out with your fantasy secretly.....this is not healthy or safe for your relationship. Even if it never goes any further the fact is you are keeping something from your partner that could be very painful for him.
You know the relationship you are having with the other guy is wrong.........I do not have to tell you this. I can sit here and tell you it must stop now and to go to your partner but you do not need me to preach to you about it. What you need is to come to the understand that what you are doing is not fair to yourself or to him. Having someone join your relationship is fine...........as long as you both are ready for it and this is what you really want. But keeping that person secret is not OK.
I must warn you that this type of play is serious and not something you want to consider without being 100% comfortable with the idea. Jumping into this activity can hurt more then help. And the fact is many people are never able to accept it and they split up. That is something you have to know and address if you choose this route.
It sounds like your partner is concnered.......and he should be. The fact is this is something that could cause conflict in your relationship so taking it slow and one step at a time is a good thing. Many new couples starting this type of lifestyle sit down and talk it out. Let him know how you feel and what you want. (Since you are not 100% sure yet you need to do some soul searching to decide what you really want). There is a big difference between fantasy and reality.........many people fantasy about this but once the action is done they feel ashamed or embarrassed. That is what you want to avoid if you decide to do this. Communication is the key..........if you want to do it you must talk about it allot. If he isn't interested you must accept that. Pushing him into it is not fair to him and will cause friction in your relationship.
Other couples start out slowly with it. Fantasy and role play first........then sit down and talk about it. Talk about how you felt and how he felt. The more you talk the better everything goes.
Keep in mind fantasy is healthy in a relationship and has its place.........but your relationship must be more then just fantasy and sex if not all you have is a bed partner instead of a life partner. As far as the other guy........you know what is right. Until you and your partner have decided what to do you may want to end this relationship. The more you talk to him the less you are talking to your partner.
No waste what so ever.......if you are not satisfied that you have been helped I am more then happy to continue to help
To tell him or not.........that is difficult. Only you know what is best in your relationship. Is this going to hurt him? Yes. Is this going to ruin your relationship? Maybe. But trust and honesty is a big part of any relationship as well......if you can not be honest with him and he can not trust you then what do you really have?
As far as the leaving you alone all week.........is this a issue for you? If so then you will need to tell him what you need. If he is not willing to give you what you need you must then decide what is best for you and go with what you want and need.
I can not tell you if being honest is the best thing for you........I can tell you that if this is going to work out that you will need to either end the relationship or tell him the truth, otherwise this secret will fester forever in your relationship.
As women age their sex drive speeds up.....the menstruating time increase the release of estrogen which is known to increase the sex drive. This is normal but if you feel it is to much you can talk to your doctor about helping to regulate your hormones more.
As for what you have to lose........you are right there is a lot to lose for both of you. The real fact is no one can stop their own fantasy's you can only regulate them to be what you want them to be. In the end if this is something you feel you have to do then it is time to sit down and be honest with him. Let him know that while you respect what he wants you also know what you want. Thus giving him the chance to back away and allow you time to know what you want.
Just do not push him to do something he is not comfortable doing.......in the end that will run him off faster then the action itself. If this is something you want.....take it at his pace.