This post justifies use of the terms “intrinsically wrong” and “intrinsically grave” instead of “formally wrong” and “formally grave.”
There is some plausibility behind a proposal to use “formally.” The intrinsic wrongness is said to be based on a characterization of the act. For instance, the description of masturbation is allegedly sufficient for showing its wrongness and gravity. The description could be called presenting the form of the act. From a proper description of what masturbation is, it can be determined that masturbation is wrong regardless of any circumstances, intentions or consequences of the act. From the form of the act we can deduce, in the proper moral theory, that masturbation is immoral regardless of anything else. So, why not say that masturbation is formally wrong? What is added by labelling masturbation intrinsically wrong? And, does not “intrinsic” obscure the fact that the wrongness is derived from the form?
“Intrinsic” adds that the wrongness is not in the form itself but in the reality of the act with the form. Use of “formally” can mislead us to think that an act is only formally wrong but perhaps in actuality not seriously wrong on not wrong at all. We might hear :It’s formally wrong but is it actually wrong?” For instance, saying that masturbation is formally wrong can mislead one to think that in reality some acts may not be wrong. “intrinsic” leads us to think of the wrongness as being in the particular acts. So, there is wrongness -evil- in each and every act of masturbation. We want more than a belief that masturbation is an act of a wrong type as suggested by use of “formally”.