Well, you've got your answer. The law does not require you to find it offensive, it requires the comment to be offensive, insulting or abusive. Regardless of whether the comment is, in and of itself, offensive it is certainly insulting both to gay people and, presumably, to the straight person whose acting is being called gay. It is an easy sending off from that perspective.
Now, there is the caveat: how public was the question? Were there others within ear-shot? Was the comment loud enough to be considered public? These are questions you must consider when dealing with both OFFINABUS and dissent. The requirement for publicity will vary, depending on the severity of the comment. In this case, it has to meet a very minimal criteria of publicity (i.e.: the reasonable possibility that someone could have possibly heard him say it) whereas something like "you're sh*t, ref" would require a much higher level of publicity to warrant a sending-off.
If I were to sum it up, I would say that the comment itself is entirely deserving of a sending off whether it is in Sunday League or Premier League and everything in between. Having said that, a strong caveat is in place which says that you, as the referee, should be reasonably assured that the comment was public enough that it is not going to be a major shock to anyone that he's sent off. In this example, the comment is so severe and heinous that the requirement for publicity is low: enough to mean that he can't rely on saying "I said play acting, ref" because someone else probably heard him. In other cases, the publicity must be more because the comment is less severe.
