My boyfriend proposed to me. Are we sinning if we have sex?

Answer:

"Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4).

Sex outside of marriage is a sin. A proposal to marry doesn't make the two of you married. Love doesn't make you married. Marriage is created by the covenant you make during the wedding ceremony. "Yet she is your companion and your wife by covenant" (Malachi 2:14). Therefore, having sex before your wedding means you are sinning -- specifically, you are committing the sin of fornication (or sexual immorality depending on your translation). "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10).

You need to stop sinning and get right with God.