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Your son has been exposed to adult sexual activity either in person or through porn (in various forms). My guess is that his exposure has continued in some way, which keeps the topic foremost in his mind. What he's doing is not normal child sex play, but follows more along the line of a child who has been sexually abused.
He's repeating this behavior after the warnings because it's a child's way to ask for help with something they can't handle or set right in their minds. My recommendation is that you speak with his pediatrician and get a referral to a counselor that works with sexually abused children as they're comfortable with dealing with children who's play has become sexualized. This is more than a behavioral issue that needs correction, although that is certainly one part of the help he needs. He needs help from an expert so that he can learn to vocalize what he's been exposed to.
Since he has words and more than one behavior he's repeating, my guess is that his exposure isn't to magazine porn, but either to videos or human exposure. As a four year old, he knows way too much about the mechanics of homosexual experience, and has a level of comfortability that comes from having sexual interactions normalized. He didn't come up with this by himself and is begging for help. If you don't get him the help he needs, he'll escalate the behaviors until help is forced into his world, which will come through child protective services and law enforcement.
My greatest concern is that an adult male has introduced this activity to him, which is the most likely situation as well as the most common.
My best to you.
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