My wife and i have been married for 26 yrs.we have ...

You should be happy that your wife can be so honest and open with you. But she loves you and wants to maintain her marriage with you and continue being a loving and devoted wife to you and mother to your four children.  I think she is seeking help and you can provide it for her by having her speak with a counsellor. Give her the love and support she needs.

OK, sorry, I read it again and I see now. My answer was totally inappropriate, I'm going to delete it.

Strangely enough I just had a conversation about that with a friend, it was about the opposite situation in which a guy told his wife after 4 children and a long marriage that he's actually gay and that he kept having relationships with men all through their marriage. He didn't want to be gay and kept trying to deny that part of himself, but eventually he couldn't take it anymore.

The question is this, if she has never been with a woman, it might be just curiosity and wanting to see how it'll be. Then you need to figure out if you can live with the notion of your wife have a one time experience with a woman, I know that for some people this is considered cheating and they will not live with it, some people might be OK with the notion and for some it would be a turn on. 

I think your wife's actually being pretty fair with sharing her feelings with you and not just having a go at it behind your back. I think that you should repay with the same fairness and be totally open and honest about what it is you can or can not live with in this situation. This has effect on the relationship, it changes things between the two of you, but it might not be all for the worse. I think that in different way her having the freedom to explore a fantasy might bring her more aware to her sexuality and bring the two of you together.

If you are comfortable with it, the best solution I can think of is a 3som - in which case she gets to be with a woman and you don't get to be left out, she gets to explore her fantasy in the frame of the relationship. 

Put it this way, if your wife told you that she has thought of having sex with another man, but firmly withheld for many years because she wanted to keep the marriage intact, would that unduly worry you now? Or are you worried that she might be a lesbian with all the related implications? In your case, no same sex relationship has occurred. I suspect that many people in committed relationships think of, or even vaguely desire sex with someone else from time to time. Because the "problem" has actually yet to become a real problem in your relationship, I would not stress out on it, and take you honest wife's word that she wants to preserve her marriage with you. Don't badger her on the issue!

She wants you to stay, but still wants to be with other women? Well as unhealthy for you as that is thats something that you need to ponder.

You need to do alot of weighing and measuring on your 26 yrs of marriage to come up with the right answer. Think about your children as well. What is best for you and your family. Is this healthy for them and is this healthy for you to be in a marriage where you have to share her with other women. I really don't know how to help you on this. This is your decision and your decision alone.  I know that its hard for you to make a choice, but a choice still would have to be made. If she wants to remain in the marriage then maybe counselling for the both of you would help. Maybe even a sex counselor would help as well to spice up your marriage to where you both would be satisfied and she wouldn't think about being with anyone else but you.