As stated on the Main Page, this trope is what happens when the writer tries to blend a Nice Guy and/or Innocent Fanservice Girl with The Pornomancer and/or Slut in order to avoid the negative connotations of the latter two. People who Really Get Around are typically portrayed in fiction as unchaste or villainous, and it's usually par for the course for heroic characters to treat sex seriously and carefully. Contemporary characters who have a lot of sex are usually Plucky Comic Relief, or their promiscuity is treated as a Character Flaw that runs contrary to being with their One True Love. In modern times, your average Chick Magnet or Dude Magnet is likely to be clueless to preserve their Nice Guy image. This trope, however, was much more prominent in older societies, which had different ideologies about sex. In particular, Mythology and Religion were very fond of this particular trope. (Such as Hercules impregnating King Thespius's daughters during his labors, Odysseus forced into being the lover of both Circe and Calypso, and the incident between Lot and his daughters.)
This trope is an excellent Power Fantasy because it means that the audience can enjoy reading a character who is ultimately "good" but still finds themselves in Deus Sex Machina situations. People who are meek or unassertive can put themselves into the shoes of this character, with the key to this trope being the character's attitude. It's about cases of coitus when it wasn't something the character initiated or acted upon; sex is something that just happens. If they had any hand in bringing to be, then it means they're a Casanova or actual Pornomancer. The "accidental" in this title means that sex is something which pursues them just as (or more) often as they pursue it.
This is not an easy trope to do. It requires constant Rule of Sexy to explain why Coitus Ensues and thus stretches Willing Suspension of Disbelief. Also, it helps when there is some explanation for why the character remains innocent despite all the sex they constantly have. Due to societal preconceptions, someone who constantly has sex is "promiscuous" and someone who is constantly taken advantage of sexually is being victimized. We have a trope which covers this for women, but it's seen as very unusual for a male. As such, it takes a very delicate touch to pull this trope off in a serious narrative.