Sexual Intercourse.

(SPOILER)
Paper Clip -> Friction -> Lubricant -> Personal Lubricant -> Sexual Intercourse

(SPOILER)Paperclip -> France -> Prostitution -> Sexual Intercourse

Haha I was going to go via the lubricant path as well xD

(SPOILER)Paper Clip -> Lock Picking -> Burglary -> Sexual Assault -> Sexual Intercourse

You early people took the path I would've taken xD

ROUND FOUR IS OVER. Original post updated.

(SPOILER)Foundry -> Molding -> Plastic -> Latex -> Condoms -> Sexual Intercourse

(SPOILER)Foundry > Magnesium > Plant > Latex > Condom > Sexual Intercourse

(SPOILER)Factory -> Adolescence -> Adolescent sexuality -> Sexual intercourse

(SPOILER)Foundry>Molds>Plastic>Latex>Condom>Sexual Intercourse

5

I'm sure someone else has this, it's like the only way I could find.

(SPOILER)Foundry > Zinc > Reproductive/Sex Organ > Sexual Intercourse

(SPOILER)Foundry -> Factory -> Hierarchies -> Feminists -> Pornography -> Sexual Intercourse

(SPOILER)Foundry -> Zinc -> Reproductive Organ -> Sexual Intercourse

Dammit Raven Nox! Beat me to it xD
This one was the hardest yet, as far as I can tell there's no other way to get three, I'll keep looking though...

I have no problem with matching results. Obviously, I can't check to see if you are cheating, but if you say you aren't I will trust your word. In a game like this, where everyone is trying to find the shortest route, it's highly likely that some answers will match. Don't worry about it. :)

Obviously, though, if you (one) do (does) cheat you're (one is) a massive goat buggerer. xD

Confound it all! I should've taken the shorter routes instead of checking for matches. xD

(SPOILER)
Foundry -> Metal -> Cleavage -> Cleavage (breasts) -> Sexual Attraction -> Sexual Intercourse

(SPOILER)Foundry -> Zinc -> Sex Organ -> Penis -> Sexual Intercourse