Hi, I will take a stab at it. For context, I am a mostly gay guy who likes getting BJs but has virtually no interest in giving them; not too interested in others' dicks, and certainly not unless there's an incredible physique attached. So, I dunno that I can help directly, but will try to help by way of analogies.
But as a straight man, I think if a guy offers to blow me, he is trying to convert me from a straight guy to gay or bi.
Maybe he's trying to convert you, but probably he isn't. Do you like watching two hot women making out? Suppose you do, and suppose you asked two hot straight women to make out in front of you for your sexual enjoyment. Would it mean that you were trying to convert them from straight to lesbian? Would you even care whether they were straight or lesbian? I'm gonna propose that you don't care about that; if they're letting you do the thing you like (watch while they make out), why would you care beyond that?
They don't get paid, they don't get off, and they are sucking dick and providing pleasure without receiving it.
I think they're receiving pleasure from sucking dick. Just because you wouldn't receive pleasure from doing that, doesn't mean they wouldn't. Have you ever wanted to feel up a woman? Even if it wasn't going to culminate in your orgasm? Some guys love eating a girl out, as opposed to only doing it as a prelude to intercourse. They may prefer doing both, but they like doing one anyway. Hehe, have you ever eaten dinner even knowing there was not going to be any dessert? Maybe you feel the dinner is pretty enjoyable even by itself. I don't receive pleasure from watching bowling on TV, but I understand that there are some freaks out there who do ;-)
Hope this helps.
P.S. As an aside:
Now since blowjobs are amazing, I have considered this, but with my sexual preference, most likely will not actually allow it.
But as a straight man, I think if a guy offers to blow me, he is trying to convert me from a straight guy to gay or bi.
Let's talk about these again real quick. If you like blowjobs, why wouldn't you allow it? I can think of a few of potential reasons. Is it any of these?
a) wouldn't enjoy the feeling of getting a blowjob from a guy?
b) scared of sex with strangers (dunno if these are strangers)?
c) afraid of getting too much unwanted attention? "if you give a mouse a cookie..."
d) embarrassed about getting a blowjob from a guy?
e) scared of being converted from straight to gay or bi?
So a) I think is a very real possibility. The only way to know is to try it and find out. If you decide you don't get any pleasure out of it, and you're a busy dude, then you are certainly not obligated to waste your time letting guys give you blowjobs.
For b) definitely there are health concerns. Not sure if you have these for women too, but you should have an equal fear in both cases, however powerful that may be.
For c) it's possible they will love your dick so much that you will have to beat them off with a stick, no pun intended. But you probably have that problem with women too and have figured out how to deal with it.
For d) well, that's just on you, but it's your prerogative

You might also be embarrassed about getting blowjobs from asian chicks or blondes *shrug*. No one should be judging you though, it's really in your head.
For e), if you can accept that being gay is a preference and not like a magical curse or disease (many people are confused about this), then we can substitute in another preference and see if it still makes sense. Let's say you love Fords and have no interest whatsoever in Chevys. One of the guys at your work happens to drive a Chevy and thinks they're the shit (i.e. good). He is always offering to let you to test-drive his Chevy. Note that in this hypothetical, you have never actually driven a Chevy. But you are pretty secure in your conviction that Chevys are not for you. You love Fords, after all. On the other hand, you happen to feel that test-driving cars is amazing — well, at least test-driving Fords is amazing, you've never test-driven a Chevy. So yeah you've considered his offer, but given your vehicular preference, you most likely will not actually take him up on it.
As a man who loves Fords, you suspect that if a guy offers to let you drive his Chevy, he's trying to convert you from a Ford-lover to a Chevrolet-lover. But would that really happen? Is the Chevrolet a crazy brainwashing machine. Do you lose the ability to make up your own mind about what you like or don't like? Does liking something "convert you" into anything? And if you did like test-driving the Chevy, would that be bad? Would you suddenly be a different person or fundamentally changed? And what if you more-than-liked it? What if you got out of the Chevy and were like "Oh shit, I had no idea. Forget Fords, Chevy forever!" And your buddy's like "Yeaaah!! Right?? That's what I've been trying to tell you!! Hey my shift's over, let's go to the Chevy showroom!" and you're like "Hell yeah we're going to the Chevy showroom!"
or "Hell yeah, watermelon is great, I dunno why I was so against it." I get that it is a scary idea because all the men in your family have been Ford men for generations, and some of them might be real shitheads to you if you even suggest that even *some* Chevys are well-made. But more likely, nobody is gonna sit around worrying about what kind of car you prefer. If they do, they're probably retards anyway.
*cough* Hope this helps.
-sb