Truth of the matter is a defense to a defamation claim. So the defendant raising it needs to prove to the relevant standard that what they said/wrote was true.
Other options are that they could claim they never said/wrote what was alleged. And then of course even if something negative/false was said, it would need to have harmed the reputation of the plaintiff for their claim to succeed. One of the classic topics of defamatory speech includes falsehoods of a sexual nature, so then in that context the damage is pretty easy to establish and then you are on to defenses.