As Chicup stated, a lot of guys who list themselves as "straight" are fronting, mostly because of the stigma even in the LS of Bi or Bi-curious guys.
I find that even if you see a male listed as straight in the top, you have to read through the entire profile and 9 times out of 10 if they are being deceptive, there will be hints that things might not be as they appear. One example, the most common one I see, might be the couple emphasizes again elsewhere that the male is straight, but then adds he is "open-minded" or maybe "not closed-minded". Another interpretation I think might be "He is straight, but not homophobic" (I mean why even add that as anyone willing to be doing sex acts in a room with other naked men is going to be comfortable being around other dicks even if they have no interest in interacting with said genitals?).
I list my status as straight and that is the only way I will conduct myself with other men, yet at the same time I have no problem being with a couple where the male is listed as Bi as long as he understands my status is not subject to exceptions. This has not always worked out in the past, as we had to break it off with one couple because the guy, I guess figuring that the our willingness to play with a bi-male somehow really meant I was not completely straight, kept trying to push his luck and as a result got neither me, of course, or the full swap he wanted from Mrs. Fours.