Is it normal that i can't stay erect in bed with my wife

Im 33 and have been married to my wife for 14yrs. She is very fit and active,we flirt and play with each other alot. I get very hard but when its time to have sex i go limp. I can only get erect if i imagine her with another guy,i even bought her a dildo just so i can pretend she's getting done by someone else. I can't have sex with her unless i think of her in that way,is that normal?