Hi M.,
It must be difficult to have no desire to have sex with the man you are going to marry. I haven't experienced that problem myself. In fact it is the other way around in my 18 year old marriage. But I have heard of the problem, and I am a nursing student, so I thought maybe I could help you.
The first possible reason for not wanting sex or not wanting to even think about it could be that your hormones are out of balance. You should probably see your doctor for a complete blood workup. I would check with my gynocologist first, and although this is hard to talk about, don't hold it back from your doctor. There may be an easy way to fix it.
Another reason could be the one my husband and I go through. We get so busy that we don't have quality time to spend alone together. We rush and rush all week, and then when we do have time, it feels like it is just another chore. Intimacy starts with conversation, not with just a jump into bed once a week. We have started trying to spend time together, even if it's just 30 minutes a day. One of the things we do during the week is read a devotional for couples. I highly recommend "Night Light" by Dr. James Dobson. Even if you are not a Christian, you can use this book to draw you closer together as you learn how to communicate with each other.
You haven't said how long this has been going on. I assume that since you have two kids, you have not always felt this way, although I could be wrong. But if you are only giving in reluctantly once a week, then you and your fiance should discuss why you might be feeling this way. It's no fun to have sex with a mostly unwilling partner. You should discuss your needs and his needs, and try to work out a compromise. Tell him what he does that makes you feel special, and I would almost guarantee if he steps up his efforts in making you feel special, the intimacy will come back and you will be more eager to have sex. Also remember that although this is an embarrassing subject, he is the one that you should be discussing it with. He needs to know, and although it will hurt him at first, he will come to see that the problem is not of his making, and that you genuinely love him and want to work this out.
Also, since you are not yet married it may be a sign that you shouldn't get married. If you are reluctant to have sex now, how will it be when you have been married for a couple of years? I would strongly encourage you to seek counseling before you get married to try to work this out. If everything else is good between you, counseling could help get this straightened out before the wedding and could lead to a better marriage.
I hope that some of this was helpful. I would really like to know what happens with your situation. I'm not being nosy, I just like to follow up and see how things have changed. Best of wishes to you, and I'm also praying that you will be able to talk to your soon-to-be husband and get your marriage back on track.