Wouldn't it be great if the world really worked in such a way that everyone was either completely gay, or completely straight, and all you needed was a scorecard to keep track?
Fact is, most guys are somewhere in the middle. (In the course of a lifetime, 80% roughly, have some sexual experience with both genders.) Do "straight" guys hate being hit on? Well, there's straight and then there's "straight." About 20% of them, apparently, never take anyone up on it, even if they are not offended.
Bi guys (the amazing invisible category, even though it is larger than the other two categories combined) are what you are really talking about, and, yeah, some of them like it when another guy makes a polite offer.
Be flexible in your thinking. It helps you keep from getting hit.