This means that for most of them, it must be pleasant. They must be able to joke around and even, occasionally, flirt. If you try to turn every social interaction into a perfectly professional unfriendly and unsexualised environment, you will very quickly find that you are the only one left.
This is even more relevant in a small startup. In my experience, the worst kind of work environment is a startup trying to act like a big multinational. None of the perks, but all of the soulless experience.
I have nothing against firing someone for bad behavior. However, with the little info you provided us, there is nothing serious.
Make sure the new employee knows that her colleague didn't like his approaches and that it won't happen again.
You should also try to get both sides of the story.
Finally, you did say this:
> The employee in question has made it clear that it's not a big deal and she knows how to deal with it (...)
How is she going to react if you fire his ass? Wouldn't it be acting like she can't take care of herself?