To me the comparison doesn't really hold up either. You can let people see your pierced nose but not pierced intimate areas for example - wouldn't generally be considered "the same".
The comparison did cause me to wonder though, if it is rude to send an ultrasound because of what some people see/feel they see in it, would it be rude to send a card with a print of Georgia O'Keeffe's flowers? I mean, some people look at an ultrasound and see a baby, the miracle of life, etc. Others see a uterus, somebody's inside's. With Georgia O'Keeffe some people see beautiful irises, others might see female genitalia. So would it be rude to risk it? Is the person sending the card obligated to know what/how others might see an image?