Are guys who get blowjobs from guys straight?

A classic study is by Laud Humphreys, "Tea Room Trade: Anonymous Sex in Public Places."

Humphreys, a sociologist and Episcopal priest, spent hours and hours hanging out in public men's rooms, also known as tea rooms, observing anonymous sex among the habitues. He noted down their license plate numbers, traced them to their homes, and asked them to participate in an apparently unrelated demographic survey. (He did this follow-up, having changed his appearance, in his serious social researcher persona.) The majority of the men he surveyed were married men who identified themselves as straight.

Make of this what you will. Needless to say, no human subjects research committee would approve this research methodology if it were proposed today.