OP, let me give you an alternative perspective here, as previous posts have mostly dealt with morality and logistics of having children and being a family member of a convicted rapist.
-
no relationship is perfect. What you feel is perfect now simply has not shown it’s imperfections. I’d consider this revelation about convictions and the denial / blocked out truth to be pretty big imperfections, even though they’re nothing to do with the chemistry / humour / love between you two, they are still very relevant given the seriousness of offence. So don’t let yourself be swayed by how perfect you find your relationship to have been in the past 12 months. It will change as you both grow and change because you are very young.
-
also along the lines of your age, you have very little resilience / life experience that comes with regularly facing shitty challenges that the older people have: your tenderness is making you very soft-hearted, empathetic and tolerant. I believe someone in your situation who is about 10 years older who is really planning for kids soon would have much less issue with cutting ties and running far away.
If you love him very much and cannot see yourself with anyone else, then you are going to be taking on the mental burden of a. Handling your father in law with caution at all times once you have kids
b. Dealing with any problems that your boyfriend will develop if he finds out about the conviction / if he sees the truth and has to confront his own memories of his childhood
c. Protecting your children at all times and costs. As PP said, what if you became unable to care for your children / died? Then the kids will end up with their father and grandfather. It is a huge worry although perhaps the risks are not Sky-high, you just can’t predict the future.
Taking all this on by staying with your boyfriend will demand a lot of you, it will need you to leave the innocent world views behind and start looking at everyone with suspicion. It will change you and you will carry this for as long as you are living with him.
I hope you can see that from an outsider’s point of view.
I believe you are going to be able to find good relationships in your future, without feeling that you’ve left your one true soulmate. I don’t fully believe that there’s only one right person for each of us.
If you were my younger sister I’d drag you out of there immediately, even if you hated me for it for the rest of your life.