A character from the movie Mean Girls who's best friend is Damian, a very bodacious gay guy. She does not identify as a lesbian but she definitely looks and acts like one. Just watch the movie I could write a book about Janis.
The act of intentionally sabotaging and destroying another person's otherwise happyrelationship due to desire for that person's significant other (which sometimes can be aided by unhappiness in the offender's current relationship). This act may or may not include actual infidelity with the desired other.