I went to the beach with my girlfriend
A person with a girlfriend wouldn't feel the need to brag about it on an internet forum, leading me to believe you really don't have a girlfriend, and you likely did not go to the beach. That is unless you consider your mother your girlfriend.
You never even had a girlfriend!
How would you know he never did? If you had simply defened yourself then your point would possibly retain some validity and people would still be capable of conceiving your having a girlfriend. Instead you choose to attack him about his accusation thus exposing your obvious state of not having a girlfriend.
[edited to include the original quote of what he said]