Because things like infidelity don't exist and you're ready for kids the second the wedding band hits finger.
I have no problem with people who are abstinent, who expect the same from their partners, or recommend it to others (in the right context, it's not that much different than recommending any other kind of birth control). People who advocate for it to the detriment of other forms of birth control are myopic jerks who are usually shooting their own cause (I.E. preventing teen pregnancy, STDs, etc) in the foot. If you teach that abstinence is the only fool proof form of birth control but here's how to use other methods effectively, you're teaching facts. If you're teaching people that abstinence is the only form of birth control that works at all, you're spewing crap and contributing to teen pregnancy and STDs.