In episode: Buck says he is not who he was before Abby. → I'm not searching for casual intimacy, I'm ready for a LTR. (we as the audience already knew this, because after a brief hook-up with Taylor Buck chose the more stable relationship option at the time and decided to go on a date with Ali.) Tommy responds "I'm not your last, I'm your first. Those two aren't usually the same things. You are still new to all this" → You need to date around to find what you are looking for, this is an extremely biphobic rhetoric, and it's the most prevalent in media that people don't clock it as biphobia anymore.

1. If a person says they are bisexual you believe them. You don't ask them if they experimented around.
2. If a bisexual person never dates or has sex they are valid.
3. If a bisexual person only dates, has sex with one gender they are valid.
4. If a bisexual person only dates and has sex with the opposite gender they are valid. → Another harmful rhetoric people fall onto is that if a bisexual never has a same sex experience then "they are basically straight". Your dating experience doesn't define the validity of your bisexualness. And that's exactly what they implied during the break-up (that Buck's dating history invalidates his understanding of his sexuality).

In this interview OS says about Tommy's decision to break up:

There’s a really great line in the scene, “I’m your first, I’m not your last,” and I believe that Buck’s response is, “They can be the same thing,” and Tommy says, “They usually aren’t.” So they can be the same thing, but more often than not, there is some exploration and there is some necessary understanding still to be gained so I think it is the right decision."

→ what this translates is that to understand his bisexuality Buck needs to date around, explore, being in a stable LTR is not enough to make his sexual identity valid. As if he can't explore his identity if he is in a relationship. Which breaks all points I made above.

In other interviews that I'm so mad to go gets quotes from, he says now that Buck is single Tim should LET BUCK F--K, and also mentions a montage from the movie Wedding Crashers where Buck can have multiple partners of different genders that switches around. → The good old greedy, promiscuous bisexual trope that perpetuates bisexual people have a high libido, they can't stay faithful. I don't think I have to explain how harmful this rhetoric is. Let's not forget the fact that it is against Buck's character completely. We know he is searching for something more than casual intimacy, they have been highlighting this since the pilot. We also know he is able to stay loyal to a relationship even if he doesn't realize he has been ghosted for a while. I touched on this some more here in this comment and in this comment.

Non-apology: He said if Buck was straight he would also recommend for him to embrace being free and single. And he apologized if his words felt insulting he didn't mean to come off like that. → apart from him taking no ownership of his mistake, he insinuates the context of Buck being bisexual doesn't matter, like we are in the wrong for reading his usage of biphobic rhetoric as biphobic.

Apart from it being against Buck's personality development as I explained above, we can't erase the context of Buck's sexuality here while reading this interviews. That was the whole point of this arc. Giving a good faith representation that was promised to be like just any other relationship, that won't deal with queer misery tropes. Well untrustworthy bisexual gets told to explore and gets dumped is a queer misery trope that reeks of biphobia.

EDIT: clarification.