Why are my group text messages sent as mms?

Please can someone help, I am on a plan with orange unlimited text. This is ideal for me when using my iphone as in often have to send a group message to 40+ people due to nature of my volunteer work. I am receiving larger bills from orange detail g charges under 'other ' when I called orange they responded by saying that on an iphone if I wish to send a group message the iphone can only send the message as a picture message as its to a group of people. Surely Apple have the technology to send a group text message! That's why I have an unlimited plan?? Please help as my volunteer work is costing me a small fortune!!!