Sure, if the straight man, like Nub's friend, specified that he was able to experience sexual attraction/pleasure, but not any romantic attraction or meaning to it and eventually concluded that he was in fact straight. There is not a very good vocabulary to describe sexuality, so maybe there's a better way to describe a situation like that, but they get to decide that for themselves. Real people's experiences of their own sexuality can be messy and complicated and as long as they're not flinging poo on other people, they're allowed to experience that complication without it being shot down as "just this" or "just that", imo. Which is what it sounds like Nub's friend was saying.
Sure, figuring it out can be messy and complicated, but we're talking about situation when a person already knows he or she is sexually attracted to men, but decides to identify as lesbian or straight man anyway.
I don't believe in nonsense that "only goldstars are real lesbians". I don't think sexual behavior really matters. But I think visibility does, and actions of people that are considered to be a part of small, misunderstood and misrepresented minority can affect that visibility significantly, in good or bad way.
I just think we have some responsibilities to other people too. It's not a matter of misunderstanding, it is understandable why they are doing it, but I cannot understand this lack of consideration for other people. I had conversations with such women. They never show any care about how their actions affect other women, or they try to undermine (sometimes even encourage) homophobia toward lesbians just to justify it, like denying that anyone, women in particular, could be only into one sex.
This is why I have huge respect for women like Riese from Autostraddle. She expresses exactly that pattern ("homoromantic bisexual"), but chose to identify as queer because she understands those issues, and in addition to bringing up subject of biphobia oftentimes, she also many times spoke about erasure of women who are exclusively into women, like during discussion about de-gaying of Amy in Faking It, when lesbian-identified women who were sexually attracted to men in comments section were delighted by that portrayal, of what they considered to be a lesbian whose body responds to men against her will and turn her into Sexual Hulk (sic).