desire to have my boyfriend ejaculate inside me- is it instinct?

[wretched-snail.livejournal.com]

I get SOOO turned on at the thought of my boyfriend coming inside of me. He does too. It's the only thing I miss about being on the pill. The other day he asked if I thought it was instinctual, as in our bodies' way of trying to get pregnant.

What do you think? Does anyone else experience this?

*title edited for clarity*