Lemon Stu is a lemon fic staple. He is the evil offspring of Bigger Is Better in Bed and Chick Magnet (don't ask). He is the go-to Author Avatar for teenagers who are much more horny than they are knowledgeable about sex.
To take it to the extreme: This is The Casanova who can, and will, sleep with anyone he likes, perhaps even Anything That Moves. Anyone he wants to sleep with will sooner or later want to sleep with him, or at least will not regret it afterward — this character may commit sex that wasn't consented to before it happened, but it won't be treated as wrong. He will be able to have sex as long as he likes, just as he likes or as his partner likes (which is generally the same); there are no failures, no disappointments. If he has strange fetishes, then the characters he sleeps with will have or will get them. If anyone disapproves of his sexual activity, they will be depicted as evil.
This character can sometimes be monogamous, in which case he will either be a Relationship Sue or be paired with one.
These characters are often written from the first-person perspective if they are written in any detail; failing that, third-person limited is common.