Would it be wrong to not tell your girlfriend/fiance/wife about your vasectomy?

I went and got a vasectomy doneabout 8 years ago when I was 24. Throughout the years I have explained that if you want children then I'm not the one for you, however I don't just bust out with vasectomy. Several times I've had girls try to scare me by lying about pregnancies and I've also had women who have told me that they'll be able to change my mind. At both of these cross roads I explain why they are wrong which invariably ends the relationship. Do women really believe that I'm a liar or are they just not hearing what I am saying?