An example of a game that doesn’t do a good job with this (at least in my opinion) is Skirjasaga - Tale of a Heifer. The tone of the weight gain is just too mean-spirited to be enjoyable, and I think that tone is quite important in whether or not a player wants to engage with the fetish content. I couldn’t bring myself to continue the game because I felt like shit for failing to save her from what is, effectively, identity death. Part of this might come down to the fact the the boss in question is a mandatory fail boss, and not a particularly well done example of the kind. It doesn’t help that I played it for the first time hot on the heels of finishing DMC5, which has a really, really good example of a mandatory failure boss, where the only thing really standing between you and victory is your own skill. And needing a hand.
On the other hand, (and a different fetish, but who’s counting?) Seeds of Destiny balances this quite well, specifically in the dungeons. In dungeons specifically, becoming too pregnant is a fail-state. But, it’s not a permanent one (usually!), and there are often hidden bonuses to unlock from failing too many times. In addition, even if you play the dungeons perfectly, you still end up engaging in the fetish via interacting with NPC’s.