I always thought that guys had a harder time with MTFs because we aren't being true to the team, but all the men I know have been accepting except one or two. For them I think the problem has been that their worldview includes only two immutable genders.
Often, people don't like their worldviews brought into question. If your view of the world is fairly simple, you don't like complications that imply the world is really more complex.
I don't talk with my male friends about getting my male parts reconfigured, partly because I think it will make them uncomfortable and partly because a lady doesn't discuss such things with gentlemen.

- Kate