Why didn't Grimes use Birth Control?

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https://www.dailymail.co.uk/tvshowbiz/article-7866627/Elon-Musks-girlfriend-Grimes-appears-announce-pregnancy-knocked-nude-photo.html?ns_mchannel=rss&ito=1490&ns_campaign=1490

He already has like 5 kids. How many more?
Why can't he just get a vasectomy and shag her till the end of their lives. Also human population is at 7.7Bil.

Scientifically speaking, what drives humans to produce more kids even with the advent of birth control? Kids are only going to cost you more and put pressure on the environment