Currently Being ModeratedJan 3, 2013 9:32 PM (in response to Marie-Bee)
The issue is as you have mentioned, it is because you are sending a message over 160 characters that it will send as an MMS.
You could try to disable MMS messaging all together (Settings>Messages) however this will prevent you from being able to send pictures/videos in future without turning it on again.
Currently Being ModeratedJan 3, 2013 9:38 PM (in response to montezooma)
May I add, that this isnt a feature of just the iPhone, and will happen with any smartphone. I think it more comes down to the carrier in how they send it over their network.