Why do men like to share their wives?

I can't explain it either. I think it's just in alot of men's biological makeup to want to reproduce. Regardless if it's just us, or more men in on the fun. I enjoy tag teaming/gang banging my girlfriend, and another girl being there just wouldn't be desirable. It's odd... Maybe I should go see a therapist about it. I know it's not ethically correct or morally right to do it. But I can't help it.