Why would he joke about having A GF?

[pinkie]

i met this guy online he 11 years older than me..and he sounded normal we chatted for about 2 weeks then i asked him if he has a facebook account he said yes so when i looked him up i found that my friend back in the uni is a mutual friend so i asked him about her he said she's the girlfriend of his friend and he saw her about 4 times and she's the one who added him, when i asked what he thought of her he said not that pretty..and i found out that he's an old friend to a relative...so he asked to see me so many times in the end after another 2 weeks a agreed to see him for a short time ...he was cute and i liked him alot...anyway we joke a lot online and he's flirty the other day i told him i had a date a CMA lecture ofcourse i was joking so he said that he has date too so asked with whom he was like with my sweet heart,,,i acted casual and said u never told me before u have a girlfriend i knew he was joking too so i asked are you planning to marry her just fooling around with her he said that marriage is a possibility... i asked where they were going he told me they going to some restaurant so i joked again and said i'll meet you there he was like great then i can introduce to her and tell your my other girlfriend i only said hahaha...now why the hell he would that?? did he figured that i liked him and he wanted to send a message that "im not interested in you" or he wanted to see my reaction??!! am so confused please help