Basically Jacob used to date Miranda and is still hung up on her. He pretty much says "Yeah she's way out of my league and too good for me......hey Shepard. Want to bang?" (not in those exact words but yeah). He also pretty much said he wasn't into it and spends the majority of the game being a ...well being Jacob.
Geth do have a *ahem* Rod as I call it, it's under the plating that covers their crotch, apprently the Geth modifyed themselves when quarian females and geth showed affection toward each other. ever since then the geth just kept the modifacation.