When you are just so bored, that you get a random (usually unwanted boner) The infamous "Boredom Boner" can appear at any time.
Mitch: "Dude... WTF?!?"
Alec: "What?"
Mitch: "You have a boner..."
Alec: "Don't worry. It's just a boredom boner it'l go away as soon as we do something."
Mitch: "Oh cool."
The uncontrollable urge to reproduce that consumes a female while she is holding a baby. This gut instinct/desire is completely independent of her true thoughts on having (and raising!) children.
Meredith never wanted kids, but every time she held her niece, her mind went blank and her ovaries starting churning--all was complete and perfect in the world. Truly nutty biology was at work- she was having a womb boner.
When you're driving along a bumpy road, and the constant bumping around makes your body get confused due to all the penis flopage occuring within your pants, and to your surprise you get a boner. Also known as a "brb."
While taking the buggy for a spin I got a bumpy road boner and had to lie about why I didn't want to get up from the drivers seat. "I'm just checking the gauges," I said.