Is this a Protestant/Evangelical thing? My assumption was that most Christians, even Catholics, used contraceptives when married for family planning.
Banning contraception and abortion isn't really about the desires, or day-to-day lives, of average conservative Christians, because you're right that most Christians use contraception when planning a family, just like everyone else, and abortion rates in red states are almost exactly on par with rates in other states (higher in some cases).
It's a push by the fundamentalists of the party, the zealots, and it's a lot more about creating and perpetuating permanent underclasses of people. You can't ever truly take away access either both abortion or contraceptives for wealthy people; you can only prevent lower-middle class and working poor people from accessing them. Rich and upper-middle class people will always be able to afford contraceptive care and, if necessary, have access to doctors who will perform safe, discreet abortions.
Poor people, on the other hand, are shit out of luck, and since you can't simply turn off human nature, you can't just expect entire segments of society to stop fuckin' just because they don't have access to this kind of health care, which means more middle and lower class people having babies, which traps them in vicious cycles of financial desperation, which makes it easier to exploit them as workers and keep wages low; the more kids you have to take care of, the fewer options you have, because you can't afford to be too picky about where you work or for how much when you have hungry kids at home. Most parents will do anything to make sure their kids suffer as little as possible, even if that means doing back-breaking work for shit pay; especially if that's their only option.
It might seem like a stretch to some, but think about it - if conservatives really cared about "the sanctity of life", they wouldn't constantly be fighting to cut funding for starving kids, for single mothers, and for impoverished urban centers. They wouldn't spend trillions on forever wars while saying with a straight face that the budget can only be balanced by cutting WIC, or Medicaid, or school lunch programs for little children. They wouldn't be utilizing the death penalty at breakneck pace to excitedly kill as many "undesirable elements of society", regardless of the circumstances of their conviction (places like Texas and Mississippi seem especially intent on executing mentally ill and disabled people as possible). At every turn, the conservative agenda is anti-life... except when it comes to preventing pregnancies. Once those babies are born, conservatives couldn't give two flying fucks about their well-being. So long as you're financially strapped trying to care for children you knew you couldn't afford and would have avoided having if only you'd had the means, the wealthy in this country have all the cheap, expendable labor they need.
It's not a coincidence that the wealth gap increased considerably once Reagan got into office, and it's not surprising that Republicans continue to espouse Reaganomics to this day, despite the mountains of evidence that it only benefits the wealthiest among us. It's very intentional, and a necessary part of maintaining that wealth gap is making sure the poorest among us are too busy struggling to survive to do anything other than feed the machine that preys on our desperation.