Are you rooted? Are you using Xposed? There is a module "Disable SMS to MMS Conversion" that you could try. It references Samsung ROMs in the description, but doesn't say it's for Samsung only. Sorry, I haven't tried it myself (as I never had this problem), so can't say anything from my own experience...
Optimus G F180S, Sling v2.2.2
Xperia X10 Mini, rooted stock 2.1
MeMO Pad HD 7 ME173X, rooted US stock 4.2.2
To whom it may concern: STOP nagging about pressing the freakin' Thanks button!!! It is rude, stupid, and annoying as hell. I am not an idiot who needs a huge in-my-face reminder to press it, and I will press it when I feel thankful, not because you asked for it!