Theseus had found himself in a trap, one in which he walked into purposefully for the sake of his daughter. Poseidon was on verge of firing her or making a scene (or worse), and in some way Theseus was protecting the integrity of his little girl---something Poseidon had little experience with. He knew that if he didn't comply worse was yet to come and he'd be at the mercy of a very angry Olympian. And in truth he'd been on that chopping block a long time ago when he failed Poseidon. It seemed pretty apparent that no one was on his side, and Poseidon was lashing out. He didn't have anyone else to control, and he was testing the limits of his loyalties. If Theseus didn't want to be loyal, then he would make him.
For now at least the quest was simple. He was to gather information off of Medusa. He'd taken a gamble by dropping into her office and asking questions. Nothing was wrong with his eyes, and for a moment when she looked at him he'd thought she might turn him to stone still. His heart had stopped, but only from his own anxiety. He felt really vile doing that, or sneaking around to figure out where she lived. Poseidon had other spies, but he knew Theseus had a past of being smarmy and charming. Medusa wasn't the kind of woman to charm with words, but maybe it would break down her walls a little.
Theseus didn't feel good about it though, he felt disgusting. When he showed up at Nike's she might as well have wrangled him with a rope around his neck like cattle. He couldn't lie his way out of this because he didn't want to.
"I've gotten myself in a mess." Every time he got himself in a web of crazy, he always got burned.