How to not fall in love with my co-author?

I'm afraid I can't answer your question about how to suppress romantic feelings you have for another person, for the trivial reason that humans have been grappling unsuccessfully with this question for thousands of years. The attempts to answer it have led to some wonderful works of fiction though, so I recommend that you do a "literature search" on the problem (pun intended) and maybe find some of the answers you seek.

What I can also add is that one of the most romantic love stories I'm aware of in science started off in a situation pretty similar to yours. It involved the meeting and eventual marriage of the mathematicians George Szekeres and Esther Klein. They were married for around 70 years and died of natural causes on the same day - see here, here and here for more details. So maybe falling in love with a coauthor is not something that one should categorically rule out. Of course, only you know your own circumstances and can decide whether this lesson is applicable to your situation.