Sending single SMS message to multiple contacts causes MMS

I'm experiencing an annoying (and costly) problem on my HTC Desire X. (Android 4.1.1) Every time I send a single message to multiple contacts at the same time, the message will be sent as MMS rather than SMS, even though it is a normal text message (not too many words). I am using Facebook's messenger app to send 'normal' messages as well, but I don't think it has anything to do with it. EDIT: okay, so after some testing I noticed it **has** to do with Facebook messenger's app. I went over all settings but can't seem to find anything that disables MMS for group messaging?