It's like, "Don't worry babe. I'll pull out. And if you get pregnant, we'll get married," then the unthinkable happens, and it's all, "How do I know it's mine? If it is, I'm not paying child support to you, because I won't have you screwing some other guy in an apartment that is being paid for with my money so our kid can live there!" Etc.
This is a version of what could happen to you, with details different and to allow for the passage of more time.
Basically, I have seen people (mostly females) wake up one day, and their partner of years, the father of their children, their best friend, has become this stranger who is suddenly their adversary in all things. This really does happen, and I have not been able to explain why, or hear anyone else do so. It could happen to anyone, even you.
It's like Invasion of the Body Snatchers, or that Star Trek TNG episode where these disembodied alien "escaped convicts" take over the bodies of Data, Counselor Troi, and Chief O'Brien, and "O'Brien" terrorizes his wife and baby by using them as hostages. I know lots of people on here speak Star Trek. Who remembers the episode I'm talking about?