Why, if a straight guy says does not want to share a bed with another man, would people assume he's homophobic?

Homophobia- "prejudice against (fear or dislike of) homosexual people and homosexuality".

Some men who don't want to sleep in the same as another man are no doubt homophobic.
But not all are. It's possible to be uncomfortable or uptight about homosexuality without being phobic about it. It seems like a bit of a knee jerk reaction to me.

OP- why not be up front with your friend instead of trying to lure him into something?