Good question. I think it depends on what you are trying to say. As in any verb, simple present tense indicates, among other possibilities, habitual action. "I am serious". I am a serious person, always. "I am being serious." At this moment, I am not joking.

Hi Andrey from what I know then there is NO aming areing or ising in French . By that I mean IAM doING something .That is an example of using the am with a verb ending in a ing action . So IAM speakING becomes,,, I speak , Je Parle . You aRE speakING becomes ,,, you speak ,, Vous Parlez .She IS speakING becomes ,,, she speaks ,, Elle Parle . It seams in French they are much more direct in the way they say things . You are correct I think in your assumptions that I AM beING serious , becomes I am serious I hope that does not confuse you too much , sorry if it does , but it is how I was taught by Michele Thomas