Dear Mike:
I am not yet convinced that wanting to be fucked means a person has "Turned" in the sense of no longer being straight, and why would it? Sex is not an activity which can be confined to one particular act. I believe that like a smorgasbord, I can try a burrito without turning Hispanic, or chowmein, without turning Chinese. Just because I want peas and carrots, doesn't mean I become Vegan. (please excuse the food references, I am on a diet right now!) I guess what I mean is, that tastes, (and needs) can change from time to time, and classifying me based on a particular whim I may have can lead to inaccuracy in judgement of my overall orientation, don't you think? As far as these boys are concerned, unless THEY say otherwise, I figure they think they are straight, and so do I.
And on the subject of a straight guy wanting to suck you off after you have taken care of his needs. Is it possible that a "Real Man" has an understanding of the concept of "Turnabout is fair play"? Maybe they figure they "Owe" it to you, and maybe secretly, they are hoping you might say, "no, thanks," and put them at ease, because now at least they made the gesture.
Peace