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 have a lot of sex are typically portrayed in fiction as villainous, or at least impure, and it's usually par for the course for heroic characters to treat sex seriously and carefully. Modern 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.
The Accidental Pornomancer trope was much more prominent in older societies, which had different laws and opinions about sex and "rape". 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.) Double Standard: Rape, Divine on Mortal was exceptionally common as well; after all, it's "not their fault" if they're so hot that even a god wants them.
This trope is also prominent in Japanese Media. A posited theory is that this is related to Japanese Politeness. For example, it's considered extremely rude to refuse a gift from someone in Japanese culture. It's better to accept the gift, even if you don't want it. Thus, if that "gift" being offered is their body.... You can see where this is going.
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. The easiest way to shatter a sexual fantasy is to make the audience feel guilt or responsibility. However, the less of that and the more Head-Tiltingly Kinky the scenario, the more potent the fantasy.
This allows people who otherwise inexperienced with sex or unable to relate to The Casanova to engage sexual subjects and fantasy with not much discomfort. 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 The Casanova, Really Gets Around, 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 Rule of Sexy to have the intended effect, but that becomes exponentially harder to do each time it happens. Every time the audience sees the trope (even if it wasn't in YOUR story), they will believe it less, and it thus starts to stretch 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.
Below are common excuses why an Accidental Pornomancer is absolved of their responsibility:
- Dismay: "I don't think we're supposed to be doing this..."
- Confusion: "I don't get it, what's the big deal about me?"
- Apathy: "Sure, I get laid, but it's not like it means anything."
- Deflection: "It's not my fault! I had to do it!"
- Blame-shifting: "They wouldn't take no for an answer."
- Pity: "I didn't want to break his/her heart or hurt his/her feelings."
- Reversing the question: "If someone that hot wanted you, would YOU say no?"
- Resignation: "Alright, you win again. Your place or mine today?"
- Complete obliviousness: "Yes, I slept with a different person everyday this month. Doesn't everybody?"