Having sex, I'm afraid, ranks you as a grown-up, and part of that is deciding when NOT to have sex. In a relationship, if sex is involved, there really needs to be a discussion of "Hey, you know, no contraception is 100% effective, these are my views on what I might do in the event of that, if you're not okay with that we need to not have sex." And if you think that conversation's scary NOW, it will be MUCH SCARIER to face on Day X of a missing period.
Which means if you can't have it, you need to leave. Seriously. That's not a healthy long term relationship.