Was kinda hoping there wouldn't be a bite tbh, Nanao seemed like he really didn't want it ...

Agreed

nanao would have hurt if they didn't make a mark tho just like the doctor said

He would've gone into irregular heat, yes, but because he was bitten during heat it wasn't his heart that was happy to do it, it was just his body. Now he doesn't have a choice because of it

They're actually fated pair and they're attracted to each other after getting to know each other. I bet the fuck out of my luck that Arisaka's character is very uptight, straightforward and clingy and doting af and i just cannot imgaine him cheating on nanao. Nanao on the other hand is a strong omega he clearly said it that more than anyone he is proud of being omega and more than anyone he detests being one, clearly shows that he accepts his being an omega as a whole and he detests that because of being an omega he is not going to be taken seriously, but Arisaka just proved nanao wrong and so he decided to give himself to arisaka the way arisaka has already given himself to him. So a bite is justified because they have come to a mutual understanding and have become exclusively bonded to each other.

While I agree about all of those things, especially before the bite, that still doesn't justify it. Arisaka said - just before - that he wouldn't bite Nanao and then he turned right around and did so while he was still in heat. Nanao was even thinking dejectedly about it earlier - how it would be so much easier just to be bitten than to have to suffer, even though it's really not what he wants. Arisaka may not cheat, but it's still Nanao's job that's in peril, while an Alpha job won't be affected at all by the pheromones they put out, or may even be view as more desirable to the company.
There was a misunderstanding that tipped Nanao in the direction of not wanting the bite, I'll admit, but the bigger reason was that he didn't want to be at the whim of an alpha, any alpha. The ending is almost creepy if you think too long about it, that Arisaka really did take over everything in his life.
Honestly, I think it's more the idea that Arisaka promised not to bite him but then did it anyway while Nanao was not in his right mind. If they had talked it out while he was calm and rational and then come to that decision, I would be onboard, but a crucial step was missing. I think they definitely could've come to that conclusion, but instead it was just an Alpha taking advantage of his Fated Pair's heat and not taking the high road.

But it's inevitable though, he's always in heat whenever his fated alpha is there because when he went to see a doctor it was because his body soughts his partner (and nanao obviously likes arisaka too emptionally and keeps thinking about him)i mean both are even, alpha's get in heat too whenever they're hot together. So your point is that nanao was not in his mind when he allowed arisaka to bite him? But is there other way for a pair to bite without actually in a euphoric/animalistic sex?

There were times, particularly post-coitus (or on the date they had??) when they were both in their right minds while being around each other. Just because it's inevitable doesn't mean they shouldn't've had thoughtful, mutual agreement on the topic. What could be more important to a Fated Pair than a bite?
Depending on the type of a/b/o story this is, it's entirely possible that the bite might not have worked outside of heat. But that still isn't an excuse for them to take the time when they are properly-minded to have a calm, rational discussion about when, how, and if they are truly certain they want this to happen.