Why do guy friends who get attached not want to meet up?

I spent a few months studying in France and travelled around in the country during this time. Made a few friends, gals and guys, who I got on well with, some I even met up with a couple of times in their respective cities. After I returned home from my studies we still kept in touch by email. Recently, as I was writing to not just one, but two of my French guy friends, about a possible trip to France (I did not even mention the possibility of going to their city to visit them), they wrote back to say that it would not be a good idea to meet up as they were now attached. When we met previously, these two guys were not attached. They don’t know each other as well.

I’m wondering, what’s the rationale behind not wanting to meet up? Can’t be that once attached one can’t have friends of the opposite sex? Is it an acceptable and standard French way of sending some signal? Because they are the ones who usually initiate emails with me after a period of silence. So what’s the idea here? We can write but we can’t meet? I don’t see the value of a real friendship that way.