Send SMS instead of MMS when texting multiple people
I was facing the same problem for long time on my Galaxy S3, at last i figure it out 

by default android SMS allow you to send same message to multiple people without converting it to MMS.
But when you Select the option Group Messaging under the multimedia message (MMS) setting 
it now set group messaging only for MMS

try un-checking the option 

hope this will help ( am using default sms of android)


Screenshot (click to enlarge)