First of all, let me say I dislike the term, "boyfriend." Anyone who is in a serious romantic relationship should not be called a boy, but a man. Same goes for "girls;" if you are mature enough to share intimately with another, you are a woman. That said, the English language does not provide terms understood widespread to refer a significant other without other connotations. Other words like partner, or companion have the implication of a gay relationship. While there is nothing wrong same-sex relationships, our society has a tendency to categorize and specify based on what is conventional. Other words like significant other, gentleman caller or beau are either too formal or too old-fashioned. Then there's the term lover that if used in casual conversation would raise some eyebrows because of it's association with purely sexual relationships. I find myself hard-pressed to find a word that balances the emotional maturity, level of commitment, expressed intimacy and effort required in a sophisticated young-adult/adult relationship. It just doesn't feel quite right to use the same word I used in 7th grade for my current, and life-long innamorato (Italian for "a man with whom one is in love").
But then again, a rose by any other name would smell as sweet.