he's cheating. if he's getting defensive and asking you all the questions and wondering if YOU'RE cheating, he's being that way 'cuz HE'S cheating. the cheater always thinks everyone else is cheating. sexual test messages from a married man to anyone other than his wife is inappropiate at best. disrespectful to you at worst. if he's online flirting is "innocent," ask to talk to this person yourself.
if he feels guilty, it's wrong. online flirting with a woman is NOT harmless when the man is married, period.
conversely, he's a man and he's not dead. it's okay to look, but never to touch and never to communicate with.
YOU are the ONLY WOMAN he should be flirting with and sending sexual texts to and exchanging racy pictures with.