The Reformed seem somewhat divided or just unclear on whether this verse contains moral equity. It is possible to have a law that is ceremonial yet founded on a moral reason (thus we ought not to boil a goat in its mother's milk insofar as it makes us cruel). As such, it is sometimes difficult to know exactly whether they think the verse is saying that the act is intrinsically immoral or only relatively so in some manner.
It might be taken as a ceremonial law, perhaps (if Thomas and many of the Reformed are to be believed) conveying a moral law so that though it may convey a moral truth (one ought not to touch a menstruating woman as it is unnatural) the law itself is founded in the previously mentioned ceremonial principle in 18:19 and the penalty appears to be something that God Himself carries ("shall be cut off" in contrast to "shall be put to death"; the civil magistrate would likely never know of this offense and thus God must take action; Targum Jonathan supports this saying God sends pestilence against them) out in contrast to other penalties which were punished civilly.
But more likely and more precisely, it is only relatively but not intrinsically sinful (insofar as it is prone to cause pain or maybe even some medical issues to the spouse): https://www.catholicintimacy.com/journal/can-catholics-have-sex-during-the-wifes-menstrual-period in this way, those who condemned such a thing as intrinsically immoral and unnatural such as Augustine, Aquinas, and many of the Reformed, were simply incorrect (except insofar as they had in mind certain sinful circumstances which can still occur). Instead, the act only may be immoral and unnatural in certain circumstances.