With the advent of "modern" social justice movements, I think there are groups of people who don't feel connected to the gay communities in their area--or rather, they don't feel connected to what they perceive as the gay community in their area, and thus, don't identify with being stereotypically effeminate.
I agree that the term "straight-acting" is inappropriate as it alludes to shame and concealment (e.g. I am straight-acting and therefore can "pass", and therefore you, as a potential partner should also be straight-acting, so I don't have to directly confront being openly gay/bisexual because we'll just both "pass".) It also doesn't make a lot of sense because outward expressions of masculinity are not the defining characteristic that differentiates "straight" from "gay". The defining characteristic that defines someone as "straight" or even "acting as straight" from the reference point of "gay" or "acting as gay" is the lack of attraction to people of the same gender, and possibly, the concommitant attraction to members of the opposite gender. "Straight" is just a slang term for "heterosexual". You can't "act heterosexual" as a homosexual person unless it's in the context of a movie or a play, or active deception.
"Having sex with a guy," means interacting with someone who has a dick. Even the most nellie of gays is still a guy. It all comes down to language and nuances of language. To say, "I want to have sex with a guy," when you mean, "I want to have sex with a guy who behaves in a masculine manner," does put down guys who don't behave in a masculine manner because it clumps all guys who don't behave in a masculine manner as "not a guy". That's hardly fair.
Again, the same theme crops up with this thread as with all of the race threads--preference is preference, and it's up to each individual to decide if they want to summarily rule-out an entire group of potential partners based on their biased ideas of that group(s)' characteristics, but I do think we all need to exhibit a degree of mutual basic respect for each other as human beings and avoid making people feel small for reasons that they have no control over.