Why does my penis go during anal play?

Hello everyone I am new to anal play but every time I put my dildo inside my dick goes soft. I start hard but after using the dildo for a little bit I go soft even if I touch myself with my hands. If I do anal only no penis stimulation after two minutes or so my penis gets really small I am above average size but when doing anal stuff only my penis shrinks to like 1-1.5 inches does anyone know why this is and is it a turn off if a bottoms dick is that small during sex?