Why does “your boyfriend will be mine” sound like “your current boyfriend will be mine”? Could it mean your ex-boyfriend will be mine?
Boyfriend, or girlfriend is a current status noun.
a person's regular male companion with whom they have a romantic or sexual relationship.
In this description current is explicit through the "have". The qualifier ex-, previous or old is required for to cover former relationships. More complicated circumstances might need descriptions like "on-again off-again" or the simpler "on off" if the relationship is regularly breaking up and being renewed.