MCK-Games here's a suggestion, for all it's worth. I love how you weaved options into the game, makes it easy to shape the experience to our tastes and the consequences down the branch lines are consistent with the choices made. Regarding Phillip's possibility of exploring sex with a guy, that was one choice I really didn't like to make, because it felt like a lose-lose. I'm not really into gay romance, it feels weird to steer Phillip towards rejecting a sexy and forward girl for the sake of curiosity. But the curiosity is there, I'd also like for Phillip to experiment, just not at the expense of another female partner. So how about...
... if Philip could choose the girl, but, in the ensuing conversation with Tanisha, he mentioned that he had the offer and the option to have sex with the guy? What if Tanisha teased him a little about it and found that she's turned on by that scenario, and pushed him towards trying - with her watching?
More nuanced choices: if the player so wishes, Phillip can feel attracted to the surfer boy, or not (first choice, what we already have). If not, later on, Phillip can accept Tanisha's "dare" to invite the boy to a threesome with her, or refuse it. If he accepts, it could be a pretty hot scene where Tanisha's encouragement could lead Phillip to overcome stage after stage of reluctance, giving him the option to back out at any point. Following through would give access to the rest of the bi content that's already written or planned for if he had refused the surfer girl's alternative offer, while still being in her arc.
I know this might sound like breaching your ethos of choices always leading to different outcomes, but I still see it as a choice between a fetishist approach to gay sex and a romantic one, which I think is valid. Whether or not that should translate into the trans arc, I'd leave it up to you, my opinion being that they can be independent, so no change required. I see it as not unlikely that Phillip would have a more romantic relationship with his trans friend, and a more fetishist relationship with cis guys, mediated/encouraged by Tanisha.
Cheers, and thanks for the game, one of the best swinging games.