On one hand, I can see how and why people are saying what they do: this, given the standards of the current era, and because Shino seemed ready to attack someone much older than Sandai--someone who doesn't have serious interest in him, and probably wouldn't. (That's given the illegality threat--though she seemed to be trying a fair bit despite that...) In addition, it makes no sense for a man to be led by a woman--especially by what could amount to nothing more than the latter's emotions running away with her. On the other hand, I can see why Shino does what she does--and she's justified because of those aforementioned standards.
Take a look at that trap character--even "he" didn't know to keep a normal distance from Sandai, constantly trying to cross boundaries typically established between males even though "he", allegedly, is one "himself". Then you have Sandai's boss, who may or may not be serious about targeting Sandai (a joke is a joke until it's not). It is held that when someone's in a relationship, friendships with the opposite sex are unnecessary; in marriage, it's a red flag. I agree with this for multiple reasons that would make feminists and their white knights want to kill me, but one I will share is pretty simple: the way people are. If a guy and a girl get along very well, and keep close contact, it's reasonable to expect romance to bloom--this is a common expectation.
This cuts both ways, for men and women. When someone's in a relationship, the other person is something everyone else is not and provides something others don't, can't, and shouldn't. So if a Sandai gets a Shino, why would he need or even want the company of a third party like that "trap"? Shino satisfies him, and vice-versa. Doesn't mean he needs to be antisocial, but it does mean that people getting especially close to him--or trying to--conflicts with Shino's reason for being so special to him. This is why I understand Sandai's indifference toward Shino's jealousy: frankly, were I in his position, I'd enjoy the attention; like the case also appears to be for Sandai, I also don't have a need to befriend/socialize with the entire world. A romantic relationship means responsibility, not just an opportunity for sexuality: both in the relationship have a responsibility to each other, and that includes bearing themselves as if they are in one.
Finally, looking at the story, it's actually refreshing. There are lot of pretty girls like Shino--and even prettier, especially them--who treat what Shino's afraid of in exactly the opposite manner: they roll over and let it happen, sometimes trying to befriend or make a "rival" (the shounen variety) out of homewreckers. I'd rather see Shino chase everyone off than see this story devolve into another ridiculous love triangle meant to pander to various fetishes.
