It really will depend on what your beliefs are. If you believe that a woman should allow any opportunity to have all the children that she might naturally become pregnant with, than perhap you might consider it a sin to prevent that from occurring.
This is why some people do not use any form of birth control.

A tubal ligation prevents an egg and sperm from meeting in the fallopian tube. The egg will then be absorbed, which many are naturally, and the sperm will die off... which many do naturally as well.