Some possibilities:
1) They use the words "straight guys" as a euphemism, to mean somebody who is masculine, straight-acting, not out as gay in their community, and so on.
2) They have a fantasy of a straight man, and hey, if somebody responds saying they're straight and want to get it on, who cares about the technicalities? It fulfills the fantasy!
3) There are plenty of men who self-identify as heterosexual even though they have sex with men (enough of them that health officials have come up with the term MSM, or men who have sex with men, to deal with the specific problems they create that need to be addressed). If a man puts out an ad asking for gay or bisexual guys, these men typically do not respond- they don't see themselves as gay or bisexual, for whatever reason, so they gloss over it. When they see something asking for straight guys, though, that means it's for them. A sort of twisted reality, yes, but one that happens nonetheless.