Of course, I could be wrong, but a date with Beth seems to be necessary, without it it is impossible to advance through the main plot and Mia’s route.It isn't needed for her to agree to a date, just get her Affection high enough (I've verified that personally, lotta shoveling). Nor do you have to actually use the spyglass if you get it. But I believe you have to try to see it at least once in order to buy horse seed for P.G. potions. Otherwise the only way to get P.G. is buying them from Edgar. And no P.G. prevents other girls' storylines from advancing.
He's said he wants to decouple her from the Main storyline (Temple+Tower). Doing that makes pursuing her completely optional...you need the manure for other girls' quests but that's as far as you would have to go and you *can* get P.G. from Edgar. The big stumbling block IMO is finding some other reason for the MC to want to learn how to summon a critter, at about the same time she currently agrees to go on a date with you. Because the Summon spell is tied to a bunch of other things including progressing the Main storyline.
And it would be okay if everything was limited to the scene of “milking” the stallion with her hand, but then you can find out that Beth allows herself in her relationship with the horse that my wife still does not allow in relation to me (JOKE) .
I will be very glad if Tinkerer reworks these "controversial" things, because this is one of the few things that I personally did not like in this wonderful game. There is also, of course, a completely unrealistic grinding system, but you can bypass it by installing a cheat mod.