Why does my red earred slider only lay one egg?

I have to red earred sliders. One is a male and one is a female. When I saw the egg I went to go grab a box full of soil to put the egg in. When I came back my boy turtle was eating the egg. Is this unhealthy for the boy turtle? In the past my girl turtle has only layed one egg. Every time an egg is layed it is eaten by my boy turtle..... why is this? When will my turtle lay another egg? (By the way the single egg was fertile.)