Also perplexed by this idea that having normal, friendly social relations with the opposite sex somehow inherently lessens the yuri, as if it was something too fragile to survive outside grossly artificial hothouse contexts like all-girls boarding schools or something...
Well, it's a question of balance and tight plotting; the way I see it. As random example, in quite nice Cross Game we've got a baseball anime, right, and it's "a bit" about the relationship between the protagonist and the heroine, of course, but even though both go to school and generally also do other things the only scenes we ever see is them playing baseball, practicing baseball, talking to each other, or talking to other people when there's some relevance to baseball or potentially to the "main" relationship. And that series got a whole 50 episodes, so it's not even short.
This is not because baseball cannot survive outside training fields, and their perfectly normal heterosexual relationship cannot bear the strain of being thrown in between other relationships ... it's merely to tighten the narrative, to show the scenes relevant to the plot (ie winning baseball games and getting the girl). That which doesn't add anything gets cut, that's just good editing, not, I don't know, narrow-mindedness.
Now, obviously your suggestion doesn't come from nowhere and often reactions or how yuri relationships are portrayed indeed come across like "no men allowed here!" It's a problem. It's also a problem that something is in almost all cases either about a lesbian relationship or simply doesn't feature lesbians. It mostly can't, for example, be about softball and only very marginally about lesbians. It'd inevitably be a yuri manga that also features some softball. That's clearly a real issue.
And, certainly, with Rin here we have one of the few examples of a manga that at least tries striking out a tiny bit from this narrow cage. So that's praiseworthy.
But it's still primarily, in any way I read it anyways, some sort of flirtatious yuri story. The relationship between Rin doesn't feel like it'll go anywhere much. And (with the series canceled) that means it feels a bit wasted as there isn't much development - the first few pages about the "people flirt with her / he helps her out / she's too go for any confusion" would have sufficed, imo; there other half of the chapter could have been used for more than a fairly cheap cliffhanger.
Incidentally, there's this vampire series by Melody Taylor, some indie thing, which features (you'll be surprised to learn) a lesbian vampire heroine and her male mentor. It's also about getting her a girlfriend and their relationship, of course, but the "mentoring" relationship is the first one the series tackles; it's very central. However, in that case the guy helps her coming to terms with being a vampire, while she helps him reminding him that even with some tragic past life's not always all rubbish (yeah, it's a bit of a trope-ish thing; I didn't say it was an awesome series ;) ) - so giving space to this made sense as it contributes to the central narrative of the story.
Which this chapter only did in a way that could have been done more effectively, I think. So that's the critique, not there being a guy.
Mh, well, that turned into quite the wall of text. Oh well. Good thing there's so much space on the Internet ...
