TLDR: Actually, everything was foreshadowed and set-up in advance pretty well.
I realize it seems to many like the progression of the latest events seem kind of rushed or out of left field, but I actually disagree, within the realm of this being, essentially, a short story and the fact that, well, there's probably a limited supply of alternative penis models to choose from in the Daz store and related sites. I mean, it would be weird if his entire body transformed EXCEPT he still had a human penis. That wouldn't actually make any sense. And, again, the creature looks more like a reptile than anything, and you do NOT want to see an MC with a lizard penis. That would be SO much worse.
The first horror movie seems, at first, like a cheap gag, because we know there is something out there in the lake, but it also brings up the idea of mad-science induced transformations. So, it was actually a foreshadowing of what's happening to Tom. That together with him exposing himself to the green goo and feeling it "tingle" made it pretty clear he was going to suffer some effect. He's been having strange pains and problems ever since. The general idea of him transforming was set-up and hinted at before hand fairly well, if not the specifics.
Claudia has implied in the past that she has a "mommy" kink, so that's not new or surprising. She's also a fan of sci-fi and horror, so a related kink is also not far-fetched or terribly surprising. (How many Star Trek fans want to have sex with an alien, raise your hands.)
Her and Tom have been getting close enough that her commitment to John was really the only thing stopping them from getting together. She's even mentioned that she's used to getting sex more often, and she's been getting pretty frustrated about it. She basically offered to do ... something sexual with Tom a night or two earlier, just to get him to give her her wine back. Once she finally decided (a little too late) to dump John, there wasn't much of anything to stop them from hooking up. The levy broke.
When she initially sees his inhuman penis, she is drunk, exhausted, and feeling betrayed. Her first assumption is that it's not even the real Tom. So, she reacts very negatively. Once she sobers up and calms down, she not only realizes the connection to her werewolf kink, but also remembers how much she enjoyed the sex the night before. Her curiosity is augmented by already knowing she's gonna enjoy it.
Yes, they need to go for help, but they are also extremely stressed out and mentally a bit frazzled. They probably aren't really thinking too clearly. They're afraid, but also getting mixed feelings about Lucas. Should they run? Should they try to talk to him? There was even talk of trying to catch him. So, the basic plan was to go for help, but they don't even really know who to go to for help with such a bizarre situation. What they do know is, they've been wanting to have sex with each other, and that line has finally been crossed. Additionally, there's more foreshadowing in touching on the topic of "teenagers always have sex instead of escaping" as referenced in the second horror film they watched. So, fuck it, let's blow off some steam. The actual sex probably would've only taken maybe a half-hour. So, it's not like they put off their plans for some unreasonably long time.
Last but not least, if you check the Dev Notes, it's clear that this is a first time practice run for making the visual aspects of AVN's. Which means, he needed to practice doing animations for sex scenes. So, if it seems like they went from "no sex" to "a whole LOT of sex" in a short time, well, we reached the part with sex in it, and he practiced a few different poses. Seems reasonable enough to me.
But I'm STILL bitter about her taking the fall for John! Bad female protagonist! No biscuit!
