Why do my SMS messages sometimes change to MMS even when I'm not sending images?
This likely only happens when you send a particularly long group text message.
The default Messaging application in the Android OS supports sending group SMS messages to a maximum of ten contacts. The maximum size of the text message can vary depending on your device and firmware version. Any message over the maximum size (sometimes 160 characters) will automatically be sent as an MMS.
There are alternative third party applications available from the Play Store through which you can send large and multiple messages as SMS insteading of converting to MMS.