I'm going to assume you started this thread as a way to ask for advice and not as a way to seek validation for your position. So having said that....
You say you've been married 18 years and she's been like this the whole time. After 18 years together, the likelihood of her changing is very slim. My recommendation is to find ways for you to both mutually find sexual satisfaction during her periods rather than trying to change her "both get off or neither do" viewpoint.
Has she ever tried Soft Cups? They are disposable diaphragms that cover the cervix and act as a catch basin. She can wear one for penetrative sex so as to make things less messy.
Have you two tried shower sex during her periods? Great way to keep things cleaner and possibly get some oral for you.