It's not that surprising when you consider that native English speakers routinely mix up "your" and "you're". Native Chinese just aren't used to thinking about pronoun gender when speaking.

It's not quite the same, though. Native English speakers don't mix up 你的 and 你是 when speaking Chinese. Although "your" and "you're" sound the same, it seems that we still inherently consider them to be different words, and the mixup when writing is more of a spelling problem.

I can't think of a better equivalent, though. Fiancé and fiancée come close, but most English speakers don't even know that there are different ways to write that for male and female.

You're right, it is kind of surprising. I guess the spoken takes precedence over the written somehow. There's even occasionally an incorrect "it" thrown in for good measure.