Why is using the Pill acceptable in Judaism?

I just read in "To Be a Jew" by Rabbi Hayim Halevy Donin that using the Pill is an acceptable form of birth control to use, as long as you plan on having, or already have, a couple of kids at least. He said that it doesn't violate the rule that says you shouldn't waste seed, but I was thinking that it actually is wasting seed, if you know you're shooting it into the uterus of a woman on the pill. What do you think?