Sex drives vary greatly among people who have no medical or psychological conditions. It may be the case that he's just naturally wired to have a lower sex drive. Normal for my husband and I is about 1-2 times a week, on a good week. It doesn't bother either of us. We both consider that normal for us. We noticed when we had sex more because we were "supposed to," it got to be somewhat mechanical for us. I know another couple whose normal is about once a month. Part of it due to the man's depression, but not all of it. Having children, stressful jobs, and other obligations make having meaningful sex difficult.
If you've had everything tested, and you know he's not cheating, it may just be what his body and mind want right now. If you're not inclined to let this ruin your marriage, and he's not inclined or you don't want to get sex elsewhere, invest in some good sex toys and porn if you're into it. Men have been using porn to blunt their sex drives forever. It helps.
I don't know if this is what you're doing, but if you're looking for permission to have sex outside of your marriage because of this, don't ask us, ask your husband.
I know you asked the "guys" but I've been there, done that (believe it or not). This is not necessatily caused by an affair, nor it is a signal for divorce. It could be:
nagging or too much emotion (from the wife) (Read the book Proper Care and Feeding of Husbands)
Reading this book is one of the last things I'd recommend to anyone. If you're not familiar with Dr. Laura Sclessinger, she is a very controversial conservative radio host and author who professes nothing but disdain for homosexuals and anyone who doesn't live in a nuclear family. The thrust of this book is that marital dissatisfaction is based largely on nagging women who want to burden their husbands with their frivolous emotions. Though some of her ideas are valid to a point, I don't think this is where you want to be looking for advice. Instead, if you want to go this avenue, get both of you to a licensed marriage and family counselor to help work this issue out.
wife frumped out on him (you don't have pics posted, so I have no idea if this happened)
I don't know that a wife "frumping out" is a valid reason for a man to stop having sex with his wife, nor do I think that a man who truly loves his wife would consider this criteria for not wanting to have sex with her. After all, you're not having sex with the body, you're having sex with the person, the whole person you married and fell in love with. Mind you, it can't hurt if, as a woman or man, to take good care of your mental and physical health so that you are more confident and a more attractive person. I don't know, my husband loved me and loved having sex with me at 230 pounds as much as he does now many pounds lighter. Sex is better now because I feel more comfortable and he likes a new, firmer me-- but he always loved me and cherished me no matter what size. Plus, overweight doesn't equal frumpy. An overweight woman who takes care of herself is still more attractive tham a skinny woman with an unkempt appearance and poor hygeine, in my opinion.
At any rate, if everything is otherwise normal for the OP, it may just be sexual incompatibility. In that case, like anything in marriage, you've got to make compromises on both sides to make it work.
Good luck!