Not everyone thinks the same way and I think the answer to your question more or less depends on what is called a person's libido in medical terminology. A person's libido depends on many things and it changes from to time. So, that makes it very difficult to answer your question even when it is asked about a single person, let alone all people.
Philosophically, I would like to think that friendship between men and women is possible. Talking from experience, it's usually not the case for a long period of time unless both people are in a phase of their life that appreciate honest friendship, regardless of gender. In my experience, sometimes people need honest friendship. On the other hand, sometimes they need to flirt to feel more self-confident and attractive. The later one usually happens when someone is in a situation where they need to be reassured of their worth and be comforted that their future isn't completely dark. In other words, they want to prove to themselves that they aren't going to die alone.
I think I can add more to this conversation, but I hesitate to write long messages because people don't read them. So, I stop here.
it's not a first time when this question appears here. Some time ago, reading the same discussion, I really liked one comment, written in Spanish, something like: if any man is not able make friends with a woman or any woman isn't able make friends with a man, they will probably have a boring life.
Personally, I agree with @aegis, that it is not common, but definately it is possible
A good way to approach it is to set ground rules from the very beginning. It doesn't have to be awkward. When you are trying to get to know each other, mention things that involve your partner. Instead of a sheepish or awkward "I have a boyfriend", you can tell stories about your life that involve him. One of my favourite ones is how I met my wife, or how we got lost travelling in x country, etc.
Why not write a short story, asking the language partner to correct it? That is perfectly normal in the context and it also reminds them that you aren't pursuing anything romantic.
I think it depends on what you want to get from a person. In my case i wasn't interested in relationships, my friend had a boyfriend so there was no problem to be just friends. Also we were studying together this united us and we had similar interests and goals.