Why do men get upset when their wives masturbate?

I am doing a report about the health benefits of female orgasms and I am looking for serious replies only! I have spoken with many women who have told me their husbands "manhood" would be insulted if she came home with a toy and used it by herself on occasion. I would really like to get a little more insight as to why men feel this way and if it is insecurity or something else. Thanks to everyone who shares their thoughts with me.