Intent is one of the fonts of morality.
What is your intent in performing these exercises?
Is it to have a larger reproductive organ so that you can be sexually promiscuous, or a “pick-up artist”, or to indulge in an immoral act such as pre-marital intercourse?
Or (more likely, since you are posting on a Catholic forum), is it because you have real or imagined concerns about the size of your
membrum virile, and worry that this will affect your future marital life?
Or is this already an area of concern in the relationship between you and your wife?
If the first, then it is sinful. Period.
If the second or the third, you may want to (a) study the Theology of the Body, and (b) consult a physician if you have genuine medical issues (hormonal, endocrine, etc.). But I would not recommend these unproven exercises, which are of dubious efficacy and could be a near occasion of sin by tempting you into the sin of onanism.
