One contributer on Urban Dictionary says the following about the adjective seedy:
Used to described a dirty, infectious place. From the term ’seed‘ used to describe semen.
Apart from the actual definition, the statement includes a supposed etymology: seedy is perceived as alluding to sperm. I can’t judge how widespread that reading is, but this user is certainly not alone in making that connection, with two more definitions on Urban Dictionary hinting at sex.
The Oxford English Dictionary (OED) gives another etymology: „App[arently] in allusion to the appearance of a flowering plant that has run to seed.“
So we have two metaphorical shifts proposed here: „looking shabby like a plant after its vital climax“ or „involving body fluids and therefore shady“. But even if the latter didn’t play any role in the origin of the term, it may contribute to its present conception.