Any SMS over 160 characters has 2 options - sent as multiple SMS messages or sent as an MMS message. It's due to the SMS protocol, which has a limit of 140 octets per message (which comes out to 160 characters). (It's actually the same protocol that was used on pocket pagers, and who needs more than 140 characters to say "It's Joe - call me at 123-456-7890"?)
That's not the issue. As soon as I select message from the people stripe it goes to the application ready to type a message but a pop up says converting to MMS.