They promoted it somewhere else? People who'd been avoiding it for a lack of MC+Haley sex finally caught on that that was no longer the case? Could be any number of reasons. I'm curious about what you're attempting to imply here.
Except the distant dad situation, everything you mentioned leads directly to or very closely back to rape
The fact that Adriana knows about Sandra's history doesn't relate to the Haley/Klaus situation and it's second-order removed from Sandra's family history. It just strikes me as an extremely unusual thing for her to know, and were I Sandra it would make me very uncomfortable. (My suspicion is that the
actual reason she "knows" is because the MC had to have the "here's how it's gonna go down" conversation with Victor, and if he was off having sex with Adriana rather than drinking with Victor, that conversation still had to be previewed. Meta-game justification, in other words. I'd still prefer an in-universe justification, though.)
We were forced into a relationship with her, so I don't really find any discussion about how asshole-y the MC was as worth discussing.
I do, because the fact is, whether or not we were forced into a relationship with her, we
were in a relationship with her. One in which we could cheat with impunity (or not), and later one in which we were forced to cheat on her. As the story approaches the endgame the behavior of the MC matters in terms of justifying which relationships survive and in what form. No NPC other than Lisa (of all people) has expressed negativity regarding infidelity or open relationships — Haley, Adriana, and Diane all seem more or less in favor — except Sandra, who's an unknown...at least until the tiny little window of possibility if you're still on her route in the latest update. There
is, however, one character who's repeatedly expressed resistance to the idea of their love interest having sex with others, whether it's in the present or the past, and that's the MC. (And I'm not referring to the Klaus situation.)
If the MC's had sex with Adriana, Diane, and Haley (Alana means he's already broken up with Sandra, though it's not yet official), and fooled around with Lisa, but can somehow end up in an open/bearded/polyamorous/whatever relationship with Haley and Sandra, it's a failure of the writing unless it's demonstrated to us that Sandra's was okay with it all along. That's why it matters; if she's
not, then the MC having sex behind her back should have negative consequences. Which, to me, would be a savvy way of introducing gameplay and significance almost all the way back to the beginning of a game that, as you say, is essentially a VN with a couple side pieces.