Feeling unsure where you end and the other person begins. Exaggerated fear of abandonment. Psychological games and manipulation. A healthy relationship is one that allows for the individuality and growth of both persons, is open to change, and allows both individuals to express their feelings and needs. A lot of your answers seem to assume we're talking about heterosexual relationships.
All humans have the same needs for love, safety, and commitment.
Gays, lesbians, and bisexuals are no different. Sometimes that can be very nice. Other times it can be experienced as a problem. Partners in same-sex relationships must deal with the stress of homophobia, society's widespread fear and condemnation of their sexual orientation. Feeling unable to be open about one's relationship with friends, colleagues, and family can leave the same-sex couple isolated and deprived of a support network.