Supporters (so probably not most people here) are correct to be annoyed if a dev is spending time on optional content that they were lead to believe wouldn't be in the game, I think. They're paying for the dev to work on the game that they were sold on. And "most people" would certainly look at you weirdly IRL if you said you shared your girl with another dude, stop kidding yourself. Your friends would talk behind your back if they thought you were into that.
I don't really care if a game has optional content, I'm not paying and I can easily click the other option. I just think it would be fair for a paying supporter to be annoyed about this.
Yeah, pretty much just this. Dev throwing time/resources into content that was never previously advertised, and that goes completely against the main themes of the game, is a bit of a slap to the face of people that have been supporting him. From what I've read in the comments, there isn't actually a sharing scene yet, just the setup, so the time investment isn't substantial yet, but it still sets a bad precedent and leaves a bitter taste in supporters' mouths.
If dev continues to add optional non-harem/netori content to the game, it will slow down development for the core content. The problem isn't specifically that optional sharing content is "bad", but rather that that paid supporters, most of whom pay specifically for the harem/netori theme, will get less "bang for their buck". Imagine paying to see a superhero movie in the theatre, and they cut 30 minutes of the movie out and replaced it with 30 minutes of "Titanic". Sure, you could walk out when the "Titanic" part starts so you don't have to see it, but you still paid the full ticket price and only got to see 2/3 of the movie you paid to see. You'd be, quite reasonably, pissed, and so should dev's paying subscribers.
For now, it's a very minor issue, but it could be representative of a much bigger problem down the road. If the optional sharing content is being done just because a whale paid to have it added, that shows that the dev may be willing to deviate further from the original theme if more money is offered. If dev just decided he likes that content, then he should probably have waited until TA is finished and then start a new game with sharing as the main theme. That would give his paying supporters the option to stop funding him without killing development of the game they love.