How to spread my legs more

I’ve never been told this before or had this be an issue, but my current partner always tells me to spread my legs more during missionary, me on top or a position (I don’t know the name of it) where he is on his side and I’m on my back angled towards him. He gets frustrated and I’m not sure what to do. I want him to enjoy it. Is there a way to spread my legs more? And stop them from instinctively closing in as I reach closer and orgasm?