Interesting conversation, to get the straight perspective on this type of gay fantasy. I have never really thought of it as trying to convert a straight guy. For me the idea is more about me being used, like a living breathing sex toy than about the straight guy. Not unlike a sleeve or fleshlight a gay dude is likely not of real sexual/emotional interest to a guy rather the gay dude is just an object to be used. The situation would, in my mind, be similar and not impact sexual preference in any way. Just as a guy who uses a fleshlight is not more or less gay/straight. I have some friends that we joke around about this sort of thing, but it is just mutual tease to get a rise out of each other. There have been times I am sure things could have happened but nothing is ever are exciting in real life and when it comes right down to it the interest is not there and lines are not crossed. I admit, I would be totally into being a str8 guys sex toy. It is just a matter of not wanting the emotional attachments and the total feeling of being used for sex but still in control. Weird power dynamics in the situation. You would be totally an object to the guy, yet you control the situation since his cock is in your mouth and you ultimatly control when and how he cums.
Another aspect to the issue is being like opposite sex friends who hook up at the bar because they have no other love interests and want to get off. They really have no interest in a relationship, are friendly maybe even flirty but not intersted in anything more than the hookup. I have known many guys and ladies who have had zero attraction to another person onther than to get themselves off they were either a penis or vagina to them, nothing more.
I think the issue comes in not with the guys who have this as a fantasy, but the guys who have this as a mission. I could never see myself continually asking a str8 friend to let me suck him off or that I can do it way better if we did not have the type of relationship that sort of tease talk is not acceptable. For sure not if it was not welcome and made either of us uncomfortable. I have had gay guys and str8 guys do this and it is not representive of anything other than them being an ass. Same happens in str8 situations where dudes hit up on the ladies continuously that they can screw them better than thier current man. Just being an ass, not representive of most str8 guys who I am pretty sure are not pushing themselves on women.
I wonder is it partially upsetting because you are being objectified in this type of fantasy with no regard to your preferences or feelings? Being objectafied in such a manner, if unwecome, is not at the top of my list and I tend to get very upset when guys focus on my cock and continually comment on wanting to suck me off even after I've politly told them no thanks or women who continually try to "change" me with just a taste of what she can do to me. I totally get where you are coming from with the idea of just try it, you might like it, come on...that was a big chunk of my life fighting off those types of ideas when I first came out and still when I meet new people who think they can just flip my switch cause they are so damn good they can make a gay guy straight.
It is odd that we (gay dudes with this fantasy) are in essence doing to straight guys what we fight against being done to ourselves. Marginalizing our sexuality to meet the desires of others.