This behavior has nothing to do with sexual behavior.
It is a natural part of puppy and adolescent behavior as they deal with hormones. Most of the time puppies outgrow this behavior but sometimes it can become a habit and then you will want to interupt the behavior and divert the pupper onto a more appropriate behavior. A water pistol or shaker can can interupt and then use a toy to divert.
Puppies will often as the grow mount siblings or other pets (or sometimes humans) in the house as a dominance behavior and the same interuption techniques can be used to teach them that is not the desired behavior. (The other dog in the home will often correct the behavior before you even get the chance)
Don't worry you will not get up in the middle of the night to find pupper logged onto the computer so he and his fuzzy stuffed friend can enjoy some puppy porn any time soon.
Good luck with the little one, I hope you will post pictures as he grows, it always happens way too fast.