Adultery is defined as “voluntary sexual intercourse between a married person and someone other than his or her lawful spouse”1 or, to put it more simply, if one of you is married then having sex is committing adultery. It doesn’t matter if you have started a boyfriend-girlfriend relationship, if one of you is married then it is adultery.
Jesus tells us that God’s intention was for life-long relationships (see Matt 19:1-12). This is the basis of Christian marriage. Anything outside of that life-long relationship was not part of God’s intention and is sin. This means that sex with someone you are not married to — even if they are your boy/girlfriend — is sinful.
1. American Heritage Dictionary (4th edn)