I'm a man who identifies as straight, but I have had sex with other men on numerous occasions throughout my life. I don't want to live in a relationship with another man. I did, in fact, have a relationship with another man in college that started as friendship, then turned into an intimate, sexual relationship. In retrospect, I was probably in love with him, and he with me, but I determined that was not what I wanted in my life, nor how I saw myself.
So, I've had relationships only with women since then, and with one woman primarily for most of my adult life. My m2m interactions are primarily, if not exclusively, due to convenience. Simply put, I like sex more and want more of it (as in frequency) than my wife. Men have been easy, convenient, and hassle free partners for me, but I'm not looking to fall in love and settle down with a man.
I really think that the fact that I can find another man online (or in the past, in a men's room in college or whatever) who, like me, enjoys sex without the need for emotional baggage, guilt, long term commitment, jealousy, or all the other complications is THE NUMBER ONE reason I've done the whole sex with men thing. Sex with another man is just that--sex. We enjoy it, get sweaty, get off, and get on with our lives. I've had some longer term buds, which is even better. Then I just have to call him up, say "you have some time today?" and we can meet and help out each other. No wooing, begging, dinner buying, or guilt is involved.
And lest it sound less exciting, the sex I've had with other guys is typically very hot. Hell, we have the same equipment, so we both are pretty expert at how it operates. I have spent some afternoons naked in bed with another man that were mind-blowing sexually, and it's not just all what part goes where. When I'm able to connect in some other way, it's very cool, and makes the physical sex better.
So, go figure. You can define it as narrowly as you want. If you want to say that someone who has ever had sex with someone of the same gender is definitely NOT STRAIGHT, then go ahead. The experiences of thousands (millions?) of people say otherwise, as do the experts in the field. But most of us live by our own definitions for just about everything important anyway, so whatever...