CRUSHER: Perhaps it is a human failing, but we are not accustomed to these kinds of changes. I can't keep up. How long will you have this host? What would the next one be? I can't live with that kind of uncertainty. Perhaps, someday, our ability to love won't be so limited.
Crusher claims her problem with the whole matter is an aversion to sudden and unexpected changes.
Imagine you're eating chocolate cake, your favorite food, and halfway through, it begins to taste like spicy chili. After drinking a glass of water, you begin to eat the chili, deciding you like it well enough, and it turns into a sour lemon, which makes your mouth pucker. You finish the lemon and three hours later, inside your digestive system, that food becomes a peanut butter sandwich, which you are deathly allergic to. Two minutes after getting treated for your allergic reaction, you're left with an aftertaste that reminds you of peppermint. What a roller coaster!
Compound that with the person who hosts the symbiont now having their own different personality, which may or may not assert itself to some degree or other, as well as the possibility that the combination of host and symbiont may produce an amalgam that is too different from the one you knew for you to handle.
I can almost see this. When you see what she's just went through, I can feel for the poor woman. Obviously that's some crazy stuff to go through.
In fact, is it possible that she accepted and wanted the female Odan on sight, and then worried about how long this one would last and then decided to end the relationship? It's possible, but I doubt it.
The problem really starts when after her talk with Troi, being with Odan/Riker, she seems to accept the idea of changes, and the new host arrives.
The key parts are when Beverly says "send him in" and she's smiling at first--the scene is almost saying she expected, even wanted the new host to be a male. Then when she sees it's a woman, her smile obviously disappears, she draws distant, and then finally gives Odan the breakup speech.
It looks like she's disappointed the host is a female, but her speech is about 'I don't know if I can keep up with the constant changes'.
Why not just be honest and say, "I'm sorry I thought you were going to be male"?
The show would never have her say something like that.