It is not at all unusual for men to get an erection during either an enema or a colonic. Part of the reason for this is the very idea of another person nearing a man's genital area sets up sort of an autonomic nervous system reaction.
In any case, most colonic therapists and enema-giving nurses or attendants have already seen plenty of male "equipment." Your package is not likely to make much of a difference to any professional administrator.