You are welcome and thank you for your short, gracious post. The way I finally learned (still learning) to make better choices in life is to see reality the way it is– not the way I wish it to be, and not in any way that it is not.
Let’s look, as an example, at the title of this thread: “My straight boyfriend’s gay best friend– what’s going on?“- in your case, what is reality: is B really straight? No, is my answer, based on all that you shared. Is E a friend (or a best friend) to B? No, is my answer. E appears like a friend at times, but.. he is not. He appears like a father figure to B (“My boyfriend said he’s like a father figure”), and at times he appeared like a grandfather-type to you (“I was trying to look at him as a grandpa figure. My son lost all his grandparents”), but in reality, he is not a father figure and he is not a grandfather type.
How do I know? Because “E is shallow and wants them young and hot… very creepy and makes me cringe“- neither a father figure nor a grandfather type can possibly be a 73 year old man who sexually desires men just out of their teenage years, including a very young man, barely 20 (B) who looked up to him as a father figure.