I think it's really common for a man to feel not threatened by his wife sleeping with another woman but threatened by sleeping with another man.
As I and another poster said, having the same "equipment" does not make you interchanable.
If you can have the bravery and strength to do it, you can totally be monogamous and let her be polyamorous. The more I correspond with men who have cheated (not you!) the less I believe the statement "my wife has a low libido" I definitely would have been put in the catagory and the fact that I didn't want to have sex with my husband had nothing to do with my libido, it had to do with the fact that our sexual differences and problems that had built up throughout the years felt insurmountable. Maybe by deailing with you wife "sowing her oats" a bit, she can come to terms with her own sexual issues and come back to you a bit more amorous.
That said, I'm not for the boundary that a person can have sex with one gender, not the other.
I'm also REALLY not for sleeping with other people to spice up your sex life. Unless all three parties are completely aware and complicit, that sounds like USING another human.
What "medical issues"? Gynochological? Mental illness?