Lisa (The Other Side of This Life)

Lisa was a surrogate who had no idea which of three candidates was the father of her baby.

History
Lisa was chosen by Carol and Rick, who were married at the time, to be their surrogate. However, the week she was inseminated, she had sex with Rick, Doug (Carol's current husband), and her own husband, Jim, meaning that any of them could be the father of the baby.

Right after having sex, she left town, meaning she hadn't had any prenatal care. Addison Forbes Montgomery brought her in for an examination. She did an ultrasound and an amniocentesis. The ultrasound showed placenta accreta. The tests also determined that despite Lisa being implanted with an embryo made of Carol's egg and Rick's sperm, the baby had no genetic ties to Carol.

They held a meeting with all the people involved and told them the situation. Lisa asked if that meant that she could keep the baby and she was told that legally, she could. She was also told that she would be taken in for a c-section if she went into labor. Lisa said she wanted Addison to take over her medical care. ("The Other Side of This Life, Part 1")

Romantic
She is married to Jim (Season 3)|Jim]]. Despite this, the week she was implanted, she had sex with him, Rick, and Doug.