Tali and Legion conflict ME2 [Spoilers]

If you defended Tali without using either the paragon (red) or renegade (blue) option, then you did lose Legion's loyalty. Check your team status at your terminal to see if he has a yellow halo below him or not. If he's showing up as loyal, it could be that you were able to open a second conversation with Legion to regain his loyalty afterwards (by convincing him that you were basically just pacifying Tali but actually agreed with him); and you have no worries.

In fact, if he isn't loyal, you might be able to still try that option if you haven't yet... but I would first check to see whether or not your passive "mastery" skill is currently offering you the 100% P/R bonus (not the 70% one). If you're current at the 70% bonus, I would re-spec your points at Mordin's upgrade terminal and select the option that does give you the 100% P/R bonus. That 30% boost difference may be enough to allow you to pass that check and regain Legion's loyalty.

Alternatively, you could try going back to a save just before doing the loyalty mission that triggered the argument and making sure you have the 100% P/R bonus in place. That may give you enough points to pass the check for the original argument.

If you just can't keep or regain Legion's loyalty and he's the only one who isn't loyal, it's not that difficult to keep everyone alive of the SM. There is a thread here that has some great advice and the OP of that thread is an absolute expert at helping people with various LM difficulties to get through the SM with everyone alive:

bsn.boards.net/thread/241/me2-mission-guide

Thanks for the info! Yeah I checked and didn't lost his loyalty, why? Because I had that second conversation with Legion and... I had to say I lied to Tali lol (It was a renegade option) I think with that it's enough for now, everyone is loyal and my body is ready for the suicide mission.

Anyway, I will try to the Mordin's upgrade terminal option. Didn't think about it tho. Thanks again!