But the Church expects people to be perfect, and isn't particularly concerned about minimizing harm in the case that you break the rules. Picture this conversation:

"Don't have sex before marriage, and don't use condoms"

"Okay, no sex before marriage. But what if I do, then should I use a condom?"

"What do you mean, what if I do? Don't do it at all. Have zero sex before marriage. Also, don't use condoms. It's simple."

"Oh, okay. So avoid having sex before marriage, and if I do have sex before marriage, then I shouldn't use a condom."

...

The sorts of rules you cite make sense when you're trying to minimize physical harms. When your job is protecting the immortal souls of people, that's a secondary concern at best.

"Don't do something that damns your immortal soul to an eternity of torment. But if you do, make sure you wear a sweater" makes about as much sense in this context.