J.,
First there is "normal" sexuality but "we" adults aren't always that comfortable with it. As a counselor with over 10 years experience with child who have been sexually abused or have sexually abused other children, I think there is need to be concerned below. I hope all parents will look at the chart below to get an idea of acceptable and not acceptable.
First, I would be curious as a parent where my son saw this behavior to imitate. If he has seen his parents have intercourse (sorry if I offended you) or seen it on t.v. then this is most probably imitation of behaviors seen. If you do not think that he has been exposed to either t.v./movies or parents having this kind of sexualized behavior I think you should be concerned about where he saw it? Like a sitters house or something....You have to understand that I was a counselor of abused children and children with sexual offending behaviors for many years so I've seeen way more than you want to hear about. So, I am telling you where "red flags" are coming up for me. Not to alarm you but you can be proactive.
If he's seeing it at home you need to realize that children that age imitate EVERYTHING they see people do especially what goes on in the home. So, you may want to monitor his intake of visual stimulus if you get my drift.
COMMON SEXUAL BEHAVIORS
AGES 2-6
Do not have a strong sense of modesty, enjoys own nudity
Sexual play with children they know, such as playing “doctor”
Use elimination words with peers
Interested in sexual content in media (TV, movies, radio)
May explore body differences between girls and boys
Touch own genitals at home, in private
Curious about sexual and genital parts
Look at nude pictures
Touch their private parts, even in public
Interested in the opposite sex
Exhibit sex play with peers and siblings; playing “doctor”
Shy about undressing
Experience pleasure from touching their genitals
INFREQUENT SEXUAL BEHAVIORS AGES 2 - 12
Puts mouth on sex parts
Asks to engage in sex acts
Puts objects in rectum or vagina
***Imitates intercourse*****
Masturbates with objects
Undresses other people
Touches others’ sex parts after being told not to
Asks to watch sexually explicit television
Touches adults’ sex parts
Makes sexual sounds
Source of this information: http://www.ncsby.org/pages/publications/Sexual%20DEvlopme...
I know this may be alarming but don't take my word for it. Talk to your pediatrician and do some research online.
It is not expected behavior for 3 year olds to imitate sex. In my humble opinion, which you do not have to take for a grain of salt, I would be concerned about where he is seeing this to imitate it and start there. Have a nonalarming conversation with him about where he saw that and what he was doing to see if he can answer you...be very careful with his environment and get some professional advice! :)
Just like I said, IMHO!
D.