This question was a dare. Look at link: http://answers.yahoo.com/question/index?qid=20090101132951AA8tXTF&r=w#TcwnDze9B2OaT49ozOh5_jPdYLgQsGH9FACDAApjLWvUw7U63_z.
It's not a sin, because there is no God. It is being unfaithful though and will hurt your boyfriend should he find out. The only guilt you should feel is from the thought of hurting your boyfriend, so you should either tell him, or end your affair.
Yes, And it's a sin that will eventually be known by others and get around . When it does, No one will want you, Not for a long term relationship anyways . Unless you come to Jesus. God bless.