A side effect of evolution that makes the need to reproduce a lot more appealing.
The actual biological purpose served by the female orgasm is that the contractions allow the cervix to 'lap up' the deposited sperm so it can reach the egg easier.
girl: You better fertilise me this time, otherwise we'll never start a family at this rate.
man: Well when I've shot my load, you bring yourself to orgasm with this vibrator. Whilst I watch. And maybe film it to show my mates.