That's the thing I worry about most. Far too many porn games with gameplay actually fail to integrate the porn in the gameplay loop in any meaningful way - or worse, relegate the porn to a fail-state - basically forcing you to lose on purpose to see the lewds.
While I'm not too enamored with the narrative framework of Subverse (romanceable characters that also breed with every critter you come across - it would have been much better if the MC had also been female) I do applaud the general idea. The female characters need to fuck monsters and get impregnated by them so they can give birth to troops you can use in battles.
I'm still not too sure how they devs of Fallen Doll plan to meld meaningful gameplay with the porn, or if there is any plan to do so.
Their recent attempt to split the game apart into SFW and NSFW parts doesn't fill me with much confidence, and while they backed away from such an idea after audience backlash, the fact that it was even contemplated does worry me that the porn and gameplay were meant to be separate aspects.
With Valve there's basically no clear rules, or at least none that anyone has been able to pin down consistently. It seems it all depends on the person that is doing the reviewing on Valve's behalf, and whatever mood they are in when they make their decision.
The devs will only really have two options:
1. Not sell the game in countries where doing so is likely to be deemed illegal (Germany et al)
2. Bend the knee and censor the game
Option 2 is the worst possible outcome, because even though they said they could release an uncensored version on DL Site, I'm not too optimistic about their willingness and ability to maintain two separate versions of the game on two different platforms.