It's really easy for me to read Phineas as asexual, given his just like total lack of interest in girls or boys compared to the other boys in the cast, who all have various moments of immature attraction to one person or another. Baljeet has his christmas kiss thing, Buford has some kind of weird partial crush on Candace, Ferb likes Vanessa, etc. Phineas seems to like romance for other people, but not be very driven to think about it in terms of himself.

On the other hand, Isabella has a very active fantasy life and knows her own desires thoroughly, and she's still interested in dating & in Phineas when we see her as a mature teen. So she pings to me as allosexual.

So if they get together in college, I'm imagining that Phineas is super inexperienced compared to Isabella. Isabella has probably dated some other people after she gave up on waiting around for him, and I imagine she isn't shy about asking for what she wants from a partner. More than likely you end up with a situation where Phineas is Ready to Try Anything Once, ever enthusiastic for new experiences, but mostly approaching the topic of having sex for the first time as an interesting new thing to do (with Isabella who he loves spending time with.)

The problem with sex is that, like, man if you're not attracted to someone it's really hard to stay engaged. It's a lot of repetitive motion on hip and hand muscles that don't get a lot of workout otherwise, and I mean Phineas probably has decent finger strength given he's a machinist, but it's still a lot the first time you have to do it.

So he's losing steam, increasingly bored, distracted, and I'm gonna be honest with you that's not an ideal state for someone trying to maintain an erection. I told you this thought was a little cursed, I'm sorry, I warned you. Anyway, you've got Isabella increasingly frustrated because this is always how it is with her and Phineas, he gets distracted and she feels inadequate. I imagine she probably says some mean things out of frustration. But they're dating now, which means they have to fucking communicate. So after Isabella has a chance to cool down and realize she was being a bit of a dickhead, they sit down and talk it out. Phineas wants to make her happy, but he's really not into the sex thing and he doesn't think trying again is going to fix the essential problem. Isabella is into the sex thing, but more than anything she just wants to be with Phineas. For any average couple this would probably end with "okay, we just don't have sex" and roll credits.

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