Ok, some more thoughts

1) The transformation of Adrian is actually quite easy to explain. He is shy, has problems with women and our of a blue finds out all the wonders of sex. If you look from that perspective - it isn't all that surprising he agrees for everything. After all - what choice does he have ? Peeking at his neighbor ? And once you agree for one thing that was previously unacceptable for you, agreeing for further ones comes much, much easier. So I personally don't see a problem here. Those are the craziest days of his life, carpe diem so to say

2) Behavior of Christine is however rather difficult to accept. Let's face it - she arranges them to meet and talk, tries to test him and see if he is a good guy. Yet she engages in lesbianism right in front of Adrian ? And places webcams at her neighbors, wich is a criminal activity ? That is hard to buy, I'm sorry. They barely met and she seems to be MUCH more interested in testing everything else BUT Adrian. He seems to be on the second plan here, this is hard to accept. Also - placing cameras at the neighbor's is against the law and could potentially ruin her career and destroy her as a person...
3) Secret ending rocks !
I don't like to criticize without giving some possible solutions, so here is what I think

I believe that the story would feel a lot more believable and would also be a lot nicer to the player if it went a little different, yet would work perfectly well with the current characters
How ? Well, instead of putting them on a wild ride... Let them first truly meet and understand each other. Christine chose him because she believes he is a good guy - so maybe they should go and form a relationship first... And then start to spice it up
So after a few dates their relationship would be established inside the game, then the time could skip a bit and we'd land with them forming a nice couple, but with Christine looking for more, asking Adrian to spice things up and explore things together
For me such a scenario would allow to keep pretty much all the content from the game, all the 3-somes, 4-somes, lesbianism and so on without making the game hard to accept as something that could really happen
Just my 2c
