Some men consider having a vasectomy even though their partners have had bilateral tubal ligation (tubes tied).
There can be a number of reasons for this.
Some men may have more than one sexual partner.
Vasectomy is justifiable in this instance to prevent unwanted pregnancy.
Having both a tubal ligation and vasectomy may give couples a sense of commitment each other.
The risk of pregnancy following tubal ligation or vasectomy is low. Thus it is not medically justifiable as the benefit of having both procedures performed does not outweigh the risks.
Couples may have heard of others becoming pregnant after tubal ligation and would like a vasectomy to make sure this does not occur.
As discussed here, tubal ligation success rates are inferior to vasectomy. However, the risks of having an unwanted pregnancy after tubal ligation is less than the risk of the vasectomy procedure itself.