David Lucado says he didn't cheat on Britney Spears -- because they were on a break when that cheating video was made.

Lucado says that Brit dumped him on August 6th, and he was caught on tape kissing another woman on the 7th. Then, on the 8th, they reconciled. And according to TMZ, he's trying to get back in touch with his ex to explain the whole thing. In other words, he had a backup hookup ready to go and was able to fit her into a 48-hour window of bachelorhood -- and that makes it OK.

TMZ says that Britney and her conservator dad Jamie are having none of it.