The guy I am seeing just told me he is seeing another girl while seeing me. but he won't tell the other girl because he won't know how she would react.
Most Helpful Girl
It sounds like he started seeing her first. It seems who ever comes along second always gets the news of the first person they are with, and not the other way around. People fear being alone and knowing that they are being deceitful will likely run the risk of them losing their "primary relationship", which is usually Relationship #1.
Anyway, the bottom line is you are currently with someone being deceitful and is practicing their lying skills while with you. I'd take this very slowly and not get your heart wrapped up too much until he can get rid of the other girl and just focus on you. Good luck! :)1