In the seventh season, Elliot told Dr. Rebecca Hendrix that he had never cheated on his wife. It wasn't until the separation when Elliot started to check around. Elliot has had two female official partners: Jo Marlowe and Olivia Benson. While Elliot and Jo were partners, he was a rookie and already married. Jo knew well of Elliot's interrogation skills and was very happy to be working with him again- she had missed their stakeouts. Elliot and Jo did flirt and seemed close- but it never extended to an affair. Although she was curious if it had happened with Olivia: no. Elliot did care for Jo; he was devastated when she left the force to attend law school. They hadn't spoken to one another for 15 years until she was transferred to fill in as the ADA. Eliot did share many Jo stories to Olivia- but she always assumed he was talking about a man! Elliot also talked about Kathy all the time.
In the following year, Elliot was temporary partnered up with Dani Beck, a Warrants officer widow who had a reputation of being overzealous. At first, Elliot and Dani had to learn to work with one another and eventually got close. One night, Elliot, Dani and Casey went out for drinks; when it was time to go, Elliot offered to take Dani home when he kissed her. They were interrupted several times by Don's urgent calls after trying to ignore them. Eventually, Dani realized she couldn't handle Special Victims but was willing to stay for Elliot. Elliot told her that it shouldn't be that way, so she left. Elliot was sad over her departure; he thought about her when Olivia returned from her FBI assignment.
Sometime during his 2 year separation, Elliot briefly saw a friend from the Immigration Agency, Courtney Williams. After Elliot and Kathy got back together, he turned to Courtney for help for a case with Olivia. She was surprised to hear from him as it was a long time. Elliot told her that he got back together with his wife. Courtney was happy for him- she thought that might've happened. Their relationship sounded casual with no sex...but then again, we'll never know.
As for Kathy, she admitted to dating while separated. It wasn't anybody Elliot knew. He wanted to know if she slept with him (wondering if there was a possibility that he wasn't Eli's biological father- different entry) but Kathy wouldn't answer as it didn't had a point until she quickly realized why. Other than that, it was never mentioned again. Elliot and Kathy moved on forward. It never appeared that Elliot was ever attracted to anyone else while with Kathy (although I'm very sure the EO shippers would highly disagree with me but Elliot has never made any move to indicate infidelity with any women, including Olivia). His father used to cheat on his mother- there could've been a lesson to Elliot who may have witnessed his mother's pain. Elliot did had his fair share of being hit on, even was blamed for sexual harassment; but he never acted on it and made a stop on it quickly.