more options are always good for edge cases.
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some women can't take birth control pills, and the other birth control options have downsides as well.
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nothing is 100% effective, so if you want to be even more sure that you won't make babies, both pertners being sterlized is extra security.
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some men would like to be sterile but are hesitant to have a vasectomy done. They are generally simple but they don't always go well.
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for younger single men in casual encounters, you can never be sure of the other's birth control status. I'm sure there are men who would like the option to be in control of their sperm.