Maybe to further compound the fact that nothing means more to Ayumi than MC. Not even a better lookin guy with a much larger package can get in between their relationship outside of their fetish experimentation? I see Ari as a bigger threat to them as possible threat if anyone were to be one. Don't let small pp insecurities distract you from the fact that
She seems to have influence over both MC and Ayumi.
To your point though, I know in NTR games there's always a much larger penis that eventually steals MC's girl after repeatedly fucking her into falling for his larger penis. I don't see that happening in this game. I could be wrong though, and Terrace could be 1 fuck away from taking Ayumi, I just highly doubt it.
Nah - What's really going to happen is that Terrace is probably (but not certainly) fucks Ayumi in front of the MC, and as a result Ayumi and the MC got so turned on, that they go like rabbits while Terrance is sitting on the side line, wondering why Ayumi did not explode in the same way with him. Do not forget that all the Netorase is just a vehicle to spice up the sexual "ratio" between Ayumi and the MC. It only makes the bond between Ayumi and the MC stronger.
About the interaction between Ayumi and Ari. There are two possibilities: a) The interaction between Ayumi and Ari is used in the same way as the interaction between Terrance and Ayumi (thus - spicing up the love and sex life between Ayumi and the MC), or b) It is the start of a love triangle between the MC, Ayumi and Ari (personally I would not mind that one bit), leaving Terrance bungling (although I would feel sorry for Terrance). I have seen that kind of relation (the love triangle) from very close by, and all I can say that after many years that relation is still going strong (so it happens actually in real life too).
About that "big penis". Some girls will like it, but in reality most girls are not that happy with it. It can even be so painful that the intercourse has to be stopped several times, and the girl has trouble being satisfied with it. So - Having a big penis is no guarantee the girl will be satisfied beyond belief. The opposite if often more true. And in general it's (literally) a PITA. Of course this is a VN with not much connection to reality, but it would make the VN more believable if this was taken in consideration.
Anyway - That are my thoughts about it. And to be clear - This is only the way I would like to see this VN go. It's just a personal wish, and not a prediction or demand (the developer must have the freedom to do what he likes). So far I am fully aboard with this developer, and I am curious how much of my wishes will turn out to be reality. I really enjoy this VN and hope this developer will go on like this...