Yay! My OTP And to answer your question, the author herself in one of the fan books said the aren't married due to military regulations, implying they would if they had the opportunity but their relationship is strong enough on its own without the commitment of marriage. And it's hinted at in the series as well, however they never come out and say it ya know? But omg, your drawing is so gorgeous!
This pairing is shipped by Arakawa so HARD, it sometimes stings the eyes. However, as far as I'm aware, they are as cannon, as possible in that awkward universe. The author handles romance scenes VERY well
The mangaka confirmed that Roy x Riza is cannon. The only reason why they're not married by the end of the series is because both want to stay in the military service (IIRC, marriage is not permissible between members of the army).
If this isn't canon, I don't know what it is. I mean, gosh, look at those two. You can see that they're both willing to do anything for the other, they both know each other so well, and they both understand where the other is coming from. Just. Ugh. So much Royai feels I can't. Just. This is perfection.