Besides the fact that is the "point" of choosing B for most of the ppl? to keep Theseus alive?
For two reasons. First, because there's no way in hell he'll be cool with us completing our quest first. A guy comes by out of the blue trying to fuck your girlfriend, you're not exactly going to want to do favours for him. He's been a gentleman about the situation, so trying to manipulate Theseus will come off as very obvious and he'll mistrust us even more. Going along with him will show him that even though we're vying for the same girl, Ean isn't actually a bad guy.
The other reason is that Theseus strikes me as a man of honor. If this was all a ploy for him to get our guard down so he could kill us in the Labyrinth, then he's certainly one hell of an actor. He's a young man who is eager to prove himself, I don't think he's deceiving us here. He's also very confident, so the idea that he could possibly even lose this challenge doesn't even occur to him, which is why he proposed it. Think of him like you would a great fighter like Muhammad Ali when he's trash-talking - he trains so hard and he's so good that losing isn't even an option.
In my mind, the plan will be to half-heartedly engage the Minotaur so we don't get hurt, allowing Theseus to slay it and claim his woman. He'll feel like he's earned Ariadne's love, we congratulate him on being the better man, then ask for his aid in finding the statue. We earn some trust, Theseus feels like he accomplished something, and finding the statue becomes significantly easier with another bro who is now our friend helping us. Having some backup in the Labyrinth would be great.
Under no circumstances should we try to go out of our way to kill the beast. Even if we don't get hurt at all, killing the Minotaur will make Theseus murderously jealous.
Oh yeah, and I think it bears mentioning even though it's obvious: if he asks how we were able to get Ariadne to fall in love with us so quickly, fucking lie about it. We have a way with women, love at first sight, etc., anything but the truth. Telling the truth in this situation will be way worse than telling Sargon the truth was.