There is no perfect birth control method short of surgery or abstinence.
There is no perfect vaccine, either
But you take what is required to offer the best possible chance of protection so you can continue to do your job
Women subject to deployment should be offered norplant, iud or another long-lasting form of birth control, or counseled to abstain. Men also counseled to avoid sex or avoid pregnancy by using condoms.
There’s a shocker, the miltary can order you to control personal behaviors detrimental to your combat fitness.
Remember, it’s a VOLUNTEER army. Don’t like it- don’t (re)enlist.