Samantha Jones originated in Candace Bushnell's collection of essays Sex And The City. She was introduced as a powerful woman who "used sex like a man", but it wasn't until the immensely popular HBO series that Samantha became a household name.
Although she spends most of the series working her way through a series of male lovers, she does have a story arc with an artist named Maria. They have a brief, but serious and monogamous relationship. For a while, their relationship goes along well and Samantha seems to enjoy exploring her sexuality with a woman. In fact, it is one of her healthier relationships.
She never uses the word bi to describe her sexuality and Charlotte clearly lacks the language to talk about her friend's sexuality. Charlotte proclaims, "She's not a lesbian, she probably just ran out of men!" Mostly her friends are more surprised by the fact that Samantha is in a serious relationship than the fact that the relationship is with a woman.
Sadly, Samantha and the crew can't seem to find the language to describe a woman who is attracted to men and women. Instead of calling her bisexual, they decide that Samantha is suddenly a lesbian. The bi representation becomes more problematic when Samantha decides that she misses men. The couple tries to work through this, but Maria feels that Samantha has intimacy issues leading them to break up.
The good news is that Samantha never denies her attraction or love for Maria, nor does she regret her same-sex relationship.