I think generally you are considered lovers once you are having sex. They're not mutually exclusive terms though! Once you become lovers, you're still gf and bf. I don't think you can equate a purely sexual relationship with no emotional attachment as lovers though.
lovers implies that there actually is emotion involved...
As shakespeare said in Romeo and Juliet, what's in a name? you can be bf/gf and still not love each other, and be cheating behind each other's backs...
Lovers can imply just sex. A boyfriend or girlfriend *should* have a more committed, emotional aspect.
By b.fs can be lovers, good ones too!
Or the other way around. Or both the same thing.
If one is talking about a "lover" when they are married or should not be in that kind of relationship, it usualy means someone to escape with in a physical and emotional sense